Age, sex, breed, and reproductive data were obtained on canine and feline populations in Alameda and Contra Costa counties, California. The demographic data were determined as of Dec 31, 1970, the reproductive data for all of 1970. The total of 27,076 households interviewed represented 5% of all households in the area. The canine and feline populations were estimated as 224,815 and 151,176, respectively. There was 1 dog for 7.3 persons and 1 cat for 10.8 persons. Almost half of the households included at least 1 dog or 1 cat. From the proportional distribution by age, it appeared that growth in the canine population may have peaked in 1968, with subsequent declines in pups entering households in 1969 and 1970. It was found that 47.8% of bitches and 64.6% of queens were neutered. Intact bitches of all ages averaged 0.2 litters each, those of queens, 0.9 litters each. The highest reproducing ages for both bitches and queens was between 1 and 3 years of age. In that age range, 62.9% of all canine litters and 74.4% of all feline litters were born; there were 0.4 litters per intact bitch and 1.6 litters per intact queen.