2- Bromoalkanoic acids and methyl 2- bromoalkanoates from C10 to C22 with even numbers of carbon atoms were bioassayed for their biological activity against 1st instars of Culex pipiens quinquefasciatus Say. 2-Bromoalkanoic acids from C14 to C18 and methyl 2-bromoalkanoates from C10 to C18 showed moderate or potent activity, whereas the others exhibited little or no activity. The biological activity was, in general, dependent upon the number of carbon atoms in the acids and in the acid moieties of the esters.