A series of 26 triploid foetuses was ascertained in a survey of spontaneous abortuses and the origin of the additional haploid complement determined in 21 of them by the study of foetal and parental heteromorphisms. In 17 the additional haploid set was paternal in origin, in 3 it was maternal in origin and in one the parental origin could not be determined. The best fit for the data using a maximum-likelihood method was that 66.4% of the triploids were the result of dispermy, 23.6% the result of fertilization of a haploid ovum by a diploid sperm formed by failure of the first meiotic division in the male and 10% the result of a diploid egg formed by failure of the first maternal meiotic division. The possible sources of error inherent in the technique are reviewed and our results compared with previously published data.