Population studies of the California harvester ant, Pogonomyrmex californicus (Buckley), were carried out in the summer of 1967. Colonies were excavated to obtain information on population size and structure. The number of workers remained rather constant at about 4500 per colony. Winged reproductives were found in late June just before they swarmed July 7. Estimates of population density were made using the mark-recapture technique with Phosphorous-32. These estimates underestimated the actual population density by 83 to 92% and violated several assumptions of the mark-recapture technique, indicating that this method is unsuitable for population estimations.