Laboratory selection of Culex pipiens larvae by fenitrothion pressure for 24 generations resulted in 25.67 fold increase in resistance to this compound. Adults resulted from the selected larvae also acquired resistance (32.3 fold) to fenitrothion. Stability of resistance was tested and indicated that regression in resistance only occurred at lower level that regression in resistance only occurred at lower level of selection. Cross resistance to malathion and DLD slightly increased by 2.1-fold and 1.25-fold respectively that may be manifestations of vigour tolerance. It is considered that the positive results obtained here increase the probability of development of such resistance in the field. However, results of cross-resistance are encouraging as they indicate that eventual development of resistance to fenitrothion does not preclude the use of other insecticides against the resistant population.