Interspecific aneuploids were made in the genus Cucurbita by pollinating the autoallotroploid (diploid C. moschata – haploid C. palmata) with pollen from diploid C. moschata. Seventy-eight, or 0·36%, of the potential ovules showed some degree of continued development after pollination. The high frequency of continued ovule and embryo development indicated a preferential distribution or loss of C. palmata univalents during meiosis in the autoallotriploid and that megagametes aneuploid for more than one C. palmata chromosome were capable of initiating continued ovule development after fertilization. Lethal effects of interspecific aneuploidy were expressed by complete embryo abortion to sterility of mature plants. Nine plants developed to maturity and seven were cytologically identified. Four were trisomic and two were monosomic for single C. palmata chromosomes, i.e. they were 2n + 1 and 2n + 19 (n = 20) respectively. One plant was 2n and phenotypically identical to the C. moschata parent. One 2n + 1 plant was fertile and the single C. palmata chromosome was transferred in the succeeding generation.