Different intensities of 5 environmental variables present during larval development were evaluated to determine their effect on the susceptibility of Culex tritaeniorhynchus to infection with West Nile virus. Adult age also was tested as a factor influencing susceptibility. In addition to susceptibility, percent pupation, median pupation time, percent emergence and wing length of adults were recorded as indicators of stress during larval rearing. Susceptibility was determined using a membrane feeding technique by simultaneously exposing adults from all treatment levels for a single variable to the same dose of virus. For the adults age experiment, ID50 values were determined for each age group tested. Larval crowding and nutrition affected the virus susceptibility of adult females. A trend of increasing susceptibility with decreasing food levels was seen. In the larval crowding experiment, the 2 larvae/ml group was significantly less susceptible that the more and less crowded treatments. Females that were 12 days old when infected also were slightly more resistant to virus infection than the 4 and 8 day old groups.