The sensitivity of the second, third (L3), and fourth (L4) instars of Culex pipiens L. and Ochlerotatus sticticus (Meigen) to two larvicides, temephos and Bacillus thuringiensis variety israelensis (Bti) was tested with and without molting occurring in the experiments. In the first experiment for both tested mosquito species, LC₅₀ values increased from the second to the fourth instar when tested against temephos, whereas for Bti the opposite trend was observed. The highest LC₅₀ value was found for fourth instars of Cx. pipens (0.00405 mg/liter) against temephos and for second instars of Oc. sticticus (0.267 mg/liter) against Bti. The determined LC₅₀ values for the second and third instars of both species decreased with an increased number of molted larvae in the experiments with temephos. For the experiments with Bti molting did not have any significant influence on LC₅₀ values, except a small increase in toxicity during the L3/L4 molt of Oc. sticticus. These findings could help assess and define larviciding, as well as influence the quantity of larvicides needed for an efficient treatment.