The dytiscid beetle, Cybister fimbriolatus (Say), (Coleoptera: Dytiscidae), found in the temporary pools in a village of Assiut, was noted to predate on the larvae and pupae of Culex pipiens L. (Diptera: Culicidae). The daily consumption of the beetle larvae C. fimbriolatus on the 2nd and 4th instar larvae and pupae of Culex pipiens was recorded .First larva of C. fimbriolatus consumed 1.33 to 19.00 2nd instar larvae/day, 2.00 to 15.67 4th instar larvae/day, and 6.00 to 18.67 pupae/day in average mosquito. The second instar larva of C. fimbriolatus consumed from 13.00 to 23.33 2nd instar larvae/day, 8.67 to 19.33 4th instar larvae/day and 9.33 to 19.67 pupae/day. The third larva of C. fimbriolatus consumed 13.67 to 24.33 2nd instar larvae/day, 10.00 to 20.67 4th instar larvae/day and 1.33 to 24.67 pupae/day. The last result confirmed that the third instar larvae of C. fimbriolatus played a bigger role than the first larvae and second larvae.
Generally, larvae of the dytiscid beetle C. fimbriolatus proved to be an efficient predator of mosquito immature and useful in bio-control of Culex pipiens L.