Aim: To determine the prevalence of Listeria monocytogenes and associated risk factors among beef operations (cow-calf and feedlot) in central and southern California. Methods and Results: A repeated cross-sectional study where faecal and environmental samples were collected from 50 operations three times a year at different seasons was carried out. Samples were tested for presence of L. monocytogenes using a combination of enrichment and polymerase chain reaction tests. Data on putative risk factors were also collected. Listeria monocytogenes was detected in faecal samples from cows, calves and other animals on calf-cow operations at proportions of 3 center dot 1%, 3 center dot 75% and 2 center dot 5%, respectively. The organism was detected in 5 center dot 3% of cut-grass, 5 center dot 3% of soil, 14 center dot 3% of irrigation ditches, 3 center dot 1% of the ponds and 6 center dot 5% of water troughs samples. Listeria monocytogenes was less common in faecal (0 center dot 3%) and soil (0 center dot 75%) samples collected from feedlots. Conclusions: Listeria monocytogenes was present at a higher proportion among cow-calf operations than feedlots. There was no significant seasonal variation in the occurrence of this pathogen within the two types of operations. Significance and Impact of the Study: If risk mitigation strategies were implemented to reduce the public health risk these should focus in cow-calf operations.