Grapsid crab (Pseudograpsus sp.) was exposed to three synthetic pesticides (pyrethroids eg. cypermethrin, deltamethrin and fenvalerate) to observe the toxicity on this crab. The crabs were exposed to 8 concentrations of cypermethrin, 4 concentrations of fenvalerate and 6 concentrations of deltamethrin and a control. For each treatment there were three replicates and they were stocked in glass aquaria with dechlorinated tap water and aeration at a density of 1 crab/l. Cent percent mortality were recorded at 0.16 ppm and 0.004 ppm for fenvalerate and deltamethrin respectively within 72 hrs. Lethal concentrations (LC50) for cypermethrin, deltamethrin and fenvalerate to grapsid crab at 48 hrs exposure periods were 0.00332 ppm, 0.00137 ppm and 0.06976 ppm respectively; 0.00208 ppm, 0.00101 ppm and 0.04351 ppm respectively at 72 hr; 0.00068 ppm, 0.00063 ppm and 0.03020 ppm respectively at 96 hr. It was observed from the study that grapsid crabs were susceptible to all of the three pesticides, and deltamethrin was the most toxic.