This study examined the utility of Fay traps baited with carbon dioxide (Fay/CO2 traps) and ovitraps for monitoring adult Ochlerotatus sierrensis over time and space in the Coast Range of northern California. During a 3-year study in a dense oak woodland, Fay/CO2 traps collected adults from March to November at rates that were correlated with air temperature through peak activity periods ending in late June for males and late July for females. Variability in total annual rainfall did not explain the 3-fold difference in the numbers of females caught among years. Yearly collections were all male-biased, but sex ratios varied to the extent that early season densities of males did not reliably predict subsequent densities of females. Determinations of the seasonal activity periods of females by Fay/CO2 traps and ovitraps were similar except that adults were captured 2-4 wk before oviposition occurred. The availability of natural oviposition sites affected ovipositional totals into the ovitraps, precluding the use of egg counts as a direct measure of population size of females either among or within years. In a subsequent comparison of 3 sites, Fay/CO2 trap captures of females 1st peaked in mid-April at each location, but peak periods of host-seeking activity lasted 1, 7, and 18 wk in woodlands with open, moderate, and closed tree canopies, respectively. Significant differences in the densities of host-seeking females were not detected by the use of ovitraps because egg counts had similar magnitude and peaked between mid-May and mid-August at each location.