Crosby, 56, was found dead in his home in Novato in Northern California late Saturday by his roommate, said Marin County Sheriff's Lt. William Donovan. He was shot in the head, Donovan added, and a 12-gauge shotgun lay near the sofa where his body was found.
There were no signs of foul play, and his death is being treated as a suicide.
Dennis Crosby became the second of four sons born to Crosby and his first wife, Dixie Lee Crosby, also known as Wilma Wyatt, to commit suicide. Lindsay Crosby, their youngest son, shot himself to death in 1989 at age 51 after learning that the oil investments his mother had made for her four sons were worthless.
Their difficulties in growing up in the Crosby household were detailed in a 1983 book "Going My Own Way," by eldest son Gary, who portrayed his father as inattentive and cruel. Gary Crosby, who recorded a few songs with his famous father, said it was a "house of terror."
Although Dennis' twin, Phillip, criticized Gary as "a whining, bitching crybaby," Dennis himself said Gary suffered the most from their famous father, who sold more than 300 million records and made 70 films, winning an Oscar for best actor in "Going My Way."
"He got the first licking, and we got the second," Dennis Crosby once said.
Friends said he had drinking problems early in life but had been sober for some time.
"It was drink and the disease of alcohol that caused him to do this," Dennis' ex-wife Arlene told STAR in an exclusive interview.
"Over the years, I'd urged him to go to Alcoholics Anonymous, but he had gone only a couple of times. Someone has to decide for themselves that they are going to make the effort to stop drinking."
The May 4 tragedy in a California boarding house where Dennis had been living mirrored the suicide of alcoholic younger brother Lindsay, 51, who ended his life with a single shotgun blast to the head in 1989.
Arlene says Dennis moved to Novato, Calif, 18 months ago. She admits he walked out on her because she was "difficult to live with," but insists that her only desire had been to encourage him to stop boozing.
"I think he had his own pain about him," she says. "But you will not find anyone who would say a bad word about him. He was sweet, kind, gentle and a wonderful father. He had a wonderful sense of humor.
"But Lindsay's suicide devastated him. He was very close to him. For the last two years, he's been distraught. Everything builds up on him. His trust fund also ran out two years ago and he had been living on very little money."
According to Marin County Sheriff's Lt. William Donovan, Dennis was found late that Saturday night by his roommate. Arlene identified the roommate to STAR as Peter Murphy.
"They were old army buddies," she says. "They had been best friends since serving together in Germany."
Arlene met Dennis in 1963 when he worked for Bing Crosby Productions in Los Angeles. She was a secretary at the time. "We fell in love and married," she says. "We had been married for 27 years and have three wonderful daughters -- ages 30, 24 and 20 -- and he was very proud of them.
"We had lunch together just one and half weeks ago, and he was saying how very glad he was that our three children were doing so well. But it was clear that, like me, he was also very sad about our divorce.
"It's very sad. I think we both felt alone, although we still saw each other and he knew that I would always be there to support him."
In addition to his three daughers with Arlene, Dennis is the father of Denise Crosby, 33, who played Security Chief Tasha Yar in the syndicated TV series Star Trek: The Next Generation.
Denise was the subject of a sensational paternity suit filed in May 1958 by her mother, Marilyn Miller Scott, against Dennis, then 23.
The lawsuit was a major embarrassment both to Dennis -- who had married a Las Vegas showgirl two days before -- and to Bing, who publicly prided himself on bring up his boys right.
"Dennis was destroyed because his father refused to help him with Denise," says a friend.
Dennis was the second of four sons born to the legendary crooner and his first wife, Dixie Lee Crosby. The quietest of the four, Dennis joined his brothers in a nightclub act during the late Fifties, often appearing on the Ed Sullivan Show.
But Dennis always had trouble coping with showbiz. "I guess I wasn't cut out to be an entertainer," he once acknowledged. "I was always painfully self-conscious out here in the spotlight with my brothers."
Bing Crosby died in October, 1977, at the age 73 while playing golf in Spain.
Friends said Dennis had been increasingly despondent since Lindsay's suicide. Moreover, he had recently broken up with his girlfriend, Meg Muir, and had reportedly begun drinking heavily -- though he had been in Alcoholics Anonymous for years.
Dennis, who is survived by a twin brother, Phillip, was the quietest of the four boys. Though he suffered at his father's harsh hand -- e.g., belt whippings, as amply detailed in eldest brother Gary's 1983 book Going My Own Way -- Dennis once said, "I let things roll right off me. I don't stay mad long." But an indifferent show-business career with his brothers, two broken marriages (five children, one illegitimate) and several business failures later, the hurt still lingered. In the end, it was one more refrain in the sad ballad of the boys who could never sing on key for their father.