A genetic study was carried out on a neotropical fish species to illustrate the refuge theory, whose main principles are summarized. The geographical structure of the anostomid speciesLeporinus fridericiwas analysed in French Guiana and Brazil by horizontal starch gel electrophoresis. Fifteen enzymatic systems corresponding to 21 loci were studied, revealing the following particularities: (1) specimens of fish from six independent coastal rivers of French Guiana form two groups geographically situated on either side of the Kourou River; (2) two alleles can serve as geographic markers,Ldh2(l30) showing an affinity between the populations in eastern French Guiana and Brazil, andMe 1(300) being specific to the west.These observations are probably related to the refuges that were formed in the Quaternary in South America, especially on the Guiana Shield. We consider that the populations of French Guiana are separated into two groups at the boundary between the two migratory flows, one arising in the west from the refuge of the Guiana Shield and the other in the east from a nearby undetermined Amazonian refuge.