Ploidy polymorphism occurs in the hybrid offspring derived from interspecific crosses between triploid plantains (Musa spp. AAB group) and diploid bananas (M. acuminata). Therefore, Musa breeders are interested in the determination of ploidy and its effects on phenotypic expression of quantitative traits. The aim of this research was to examine the reliability of stomatal and other phenotypic traits to determine ploidy in segregating plantainbanana hybrid families. Stomatal density and size were significantly correlated (P<0.01) with ploidy, although the correlation coefficients were not high (r=-0.49 and r=0.47, for stomatal density and size, respectively). High density of small stomata was correlated with low ploidy level, and vice versa. However, stomatal size and density were also influenced by a significant genotype effect (P<0.001) within the same ploidy level. Ploidy had an important effect on fruit traits and plant height in the hybrids of 'Obino l'Ewai'×'Calcutta 4', but this was not so clear in 'Bobby Tannap'×'Calcutta 4' hybrids. 'Obino l'Ewai' derived tetraploids have medium to tall plants with large bunches and big fruits. Most of the tetraploids derived from 'Bobby Tannap' have short stature due to the gene action of the dwarf, dw, allele. Also, a few selected diploids derived from 'Bobby Tannap' outyielded their non-selected tetraploid full-sibs. In conclusion, chromosoem counting remains the only accurate proof of ploidy levels in Musa germplasm. © 1995 Kluwer Academic Publishers.