A variety of tissues from T. zillii was examined by horizontal starch gel electrophoresis to identify isozymes of potential use as genetic markers for stock identification. Techniques were assessed for 30 enzymes coded for by a total of about 60 gene loci. In each case, full details of methods are given. There is also discussion of the genetic interpretation of isozyme banding. This investigation provides a pattern for selection of isozymes as genetic markers in tilapias and related species.