Silica gel granules coated with cyclosporin A were used for intoxication of larvae of Culex pipiens. Pathological changes were evident in all tissues, characterized by the formation of minute vacuoles in the cytoplasm. Individual cells were affected to different degrees. The targets of cyclosporin A were mainly mitochondria which inflated their cristae, disintegrated and changed into vacuoles with ghosts of their structures remaining. Minor changes were evident in the endoplasmic reticulum and the Golgi apparatus. Microvilli, cell membranes and nuclear membranes were not affected, but were involved in the stages of degeneration of the killed cell.