RESUMEN
The possible juvenilizing effects of Dimilin and Altosid against larvae of Culex pipiens L. were studied. Apart from delayed lethal action, possible effect on the mean number of basal follicles developed per female has been considered. A baseline exposure was found which caused only low mortalities of the early 4th larval instar. Results showed a correlation between the overall deaths during development and the sex ratio. Higher mortality increases the proportion of females. Treatments with 0.00001, 0.0001 and 0.001 ppm Dimilin caused 9.6%, 33.6% and 18.6% increase in the mean basal follicle numbers developed per female, respectively. In the case of Altosid, these levels of increase were 23.1%, 26.5% and 23.8%, respectively, after treatments with the same concentrations.