Reproductive abnormalities reduced the percent stainable pollen, and fruit and seed set in interspecific F2 populations derived from crosses of Lycopersicon esculentum and L. pennellii but were not observed in parental lines and interspecific F1 populations. The degree to which these reproductive abnormalities were expressed in the interspecific F2 populations was affected by cytoplasm. Reproduction was impeded in interspecific F2 populations containing L. esculentum cytoplasm (F 2 Le ) by reduction in pollen production, the lack of fruit set and a high proportion of parthenocarpic fruit among plants capable of fruit set. The F2 populations containing L. pennellii cytoplasm (F 2 Lp4 ) showed a reduced frequency of reproductive abnormalities at all stages of reproductive development, resulting in higher values for percent stainable pollen, fruit and seed set and higher proportions of the F 2 Lp4 populations being capable of setting fruit or seed than F 2 Le populations. The major barrier remaining in F 2 Lp4 populations was reduced fruit set compared to parental lines. The barrier to fruit and seed set observed in the F 2 Le populations, and to a lesser extent in the F 2 Lp4 populations, occurs around the time of fertilization or early embryonic development. The effect of L. pennellii cytoplasm on barriers in the F 2 Lp4 populations is proposed to be due to an interaction between cytoplasmic and nuclear genes during fertilization of the F1 plants to produce F2 populations and may also affect subsequent generations.