To determine whether the prevalence of preprandial insemination in deer ticks reflects their local abundance, we sampled adult ticks by flagging vegetation at selected sites in eastern Massachusetts in a standardized manner. Resulting female ticks were dissected to determine whether they contained endospermatophores, and the frequency of insemination was compared to the number of questing ticks flagged at each site. The prevalence of insemination correlated closely with density of ticks. The frequency of insemination increased linearly during the Ist 2 months during the fall. The mean daily probability of insemination during this period was about 1% when, on average, about 4 ticks were flagged per minute. A predictive equation was derived via multiple regression expressing deer tick abundance as a function of collection date and insemination prevalence (P < 0.05). The frequency of preprandial mating, thereby, was correlated with the abundance of questing deer ticks. Insemination prevalence increased predictably as the season of adult activity progressed. We conclude that a season-specific analysis of the frequency of preprandial insemination provides a robust indicator of the abundance of deer ticks that is unaffected by short-term fluctuations in the weather.