Biological fitness components of a field-collected colony of Culex quinquefascialrzs Say thatwas highly resistant to Bacillus sphaericus strain 2362 (resistance ratio greater than I 63,000) after 46 generationsof selection were compared to those of a susceptible colony (CqSF) that had originated from the same parentalcohort but that had not been exposed to B. sphaericu.s. The effect of B. sphaericus on the fitness of C.r.quinquefasciatus was determined in terms of fecundity, fertility, and development time. The resistant colony(CqRL) showed significantly lower fecundity and fertility, and slower development than the susceptible colony.Development time from egg to egg showed a 20% increase in CqRL compared to CqSF The generation timeincreased from 21.6 days to 26 days for highly resistant generations ofCqRL.