The larvicidal potential of several dopamine 1-phenylisoquinoline derivatives against Culex quinquefasciatus third instar larvae was investigated in a rational search for insecticides. The results showed that these isoquinolines presented moderate larvicidal activity, that the presence of a substituent is needed on carbon 1 for such activity to be presented and that it may be possible to develop novel insecticidal compounds having potential use in controlling insects by appropriate structural modification of 1-phenylisoquinolines. This article presents a possible structure-larvicidal activity relationship for isoquinolinic compounds.