We consider the estimation of the proportion of triploids in populations of plants or animals in which diploid and triploid individuals coexist, using data from electrophoretic analysis of isozyme or microsatellite markers. Individuals that have three distinct alleles at a locus are unambiguously triploid. However, other individuals cannot be classified with certainty as diploid or triploid, unless allelic dosage can be determined reliably. This is impossible for microsatellite markers, and for many isozyme markers. We therefore present a maximum likelihood method of estimating the proportion of triploids based only on the presence or absence of different alleles.