The effect of the insecticide cypermethrin on the development of Danio rerio was analyzed. 80 recently hatched live fingerlings were used, which were placed in four Petri dishes with 20 organisms each: distilled water was placed in the control group; group 1, group 2, and group 3 were inoculated with cypermethrin at a dilution of 1 μL/L, 3 μL/L, and 5 μL/L, respectively. The fry were kept for 10 minutes and later they were transferred to other Petri dishes with water from the hatching aquarium, 24 hours later they were observed under a microscope. The control group presented normal development, however, groups 1, 2 and 3 presented damage to the notochord, when comparing the frequencies of malformations in the notochord with the control group 1 (X2=21.5384, gl=1, p< 0.05), group 2 (X2=40, gl=1, p< 0.05) and group 3 (X2=32.7272, gl=1, p< 0.05) presented significant differences with the control. However, when comparing the different dilutions, significant differences were only found between group 1 and group 2 (X2=7.0588, gl=1 p< 0.05), the latter being higher. It is concluded that exposure to cypermethrin in early stages causes damage to the notochord of Danio rerio.