The parent strain of Pectinophora gossypiella (Saund.) was col­lected from cotton fields and reared in the laboratory for several genera­tions free from insecticidal pressure. Selection of newly hatched larvae with cyanophos for 24 generations resulted in a 32.51-fold increase in resistance. Cytological behaviour of chromosomes in meiotically dividing cells was studied at the 24th selected generation. Also, the cytological aber­ration in relaxed strain for one generation was studied. Results show that the percentage of abnormal cells was 26.087 at the 24th selected generation. Relaxed strain showed a slight decrease of percentage of abonormalities (22%) as compared with the suscepti­ble strain (2.8). The most frequent chromosomal aberrations were stick­iness of chromosomes and lagging chromosomes. Irregular distribution of chromosomes was also observed in a few cells.