Summary: Milk samples were collected from all lactating cows on 60 dairies (mean number of cows/dairy, 584; range, 66 to 2,834) randomly selected from 701 California dairies enrolled in the Dairy Herd Improvement Association program. Samples were tested, by means of an elisa, for antibodies against Salmonella serogroup B, C1, and D1 antigens (somatic antigens 01, 4, 6, 7, 9, 12). Blood samples were collected from all cows with positive results and tested for serologic evidence of exposure to salmonellae. Samples for bacteriologic culture (pooled feces from 20 randomly selected calves, swabs of wet areas and feces from calf pens and dairy hospital pens, drag swab sample from wastewater lagoon, and samples of feed components) were also collected from all 60 dairies. Seven (11.7%) of the 60 dairies each had 1 sample that yielded Salmonella organisms (3 S typhimurium, 1 S dublin, 1 nonmotile Group D salmonella, 1 S derby, and 1 S oranienberg). Five of the Salmonella isolates came from the hospital pens and 2 came from calf pens. Thirty-three dairies did not vaccinate cattle against salmonellosis, and of these, 24 (72.7%) had ≥ 1 seropositive cow (titer ≥ 200), and 20 (61 %) had ≥1 persistently seropositive cow (titer for each of 2 blood samples collected ≥ 60 days apart was ≥ 200). Of the 27 dairies that did vaccinate cows against salmonellosis, 24 (89%) had ≥ 1 seropositive cow, and 21 (78%) had ≥ 1 persistently seropositive cow. We concluded that studies that use of bacteriologic culture of fecal and environmental samples to determine the percentage of dairies with Salmonella-infected cows may underestimate the true percentage.