Corydoras aeneus is an armoured catfish found in the upper amazon regions of South America. It is a member of the family Callichthyidae. Studies done with this species of fish showed that C. aeneus is a tetraploid with a chromosome number of 132 (Scheel et al. , 1972). Dunham et al. (1980) reported 120 chromosomes. The C. aeneus used in this study were bought from direct importers. We had four samples of fishes supposedly coming from Brazil (Belem), Guyana, Peru and Trinidad. During our initial studies in an attempt to karyotype aeneus we came across individuals with a highly reduced chromosome number. A diploid form of C. aeneus was discovered. Only those fishes from our Belem sample were diploid. A comparison of the diploid and tetraploid forms was done. Both forms were karyotyped, the tetraploid form of C. aeneus has 134 chromosomes and the diploid has 56. Physically both forms looked exactly the same. Morphometric as well as meristic data was collected from 131 fishes and analyzed by multivariate, discriminant and contingency chi- square analyses. The results obtained do not suggest any absolute morphological differences between the diploid and the tetraploid forms anymore than between tetraploids. / M.S.