Yield components including total tuber yield, total tuber number, vine maturity, vine vigor and average tuber weight as well as seed germination and pollen stainability were studied in 28 exact reciprocal hybrid families between 12 Solarium tuberosum Group Tuberosum (haploid) (TH) and 12 S. tuberosum Group Phureja (PH) clones in a NC Mating Design II in 4 sets of 3 by 3 reciprocal hybrids at three locations, one in Minnesota (USA) and two in Morocco. TH parents were insensitive to daylength whereas most PH clones required critical photoperiods equal to or less than 14 hours for tuberization. Mean reciprocal differences were significant for most traits at all 3 locations. Hybrids with TH cytoplasm were superior to those with PH cytoplasm by 18% for tuber yield, 21% for tuber number, and 9% for vine maturity, but inferior to hybrids with PH cytoplasm by 11% for vine vigor and 19% for time to reach 80% seed germination. Reciprocal differences were not detected for average tuber weight. Six reciprocal crosses revealed significant differences for pollen stainability but the overall mean difference (one location) was not significant. Possible reasons for the observed reciprocal differences included gamete selection, pseudo-selfcompatibility and/or interaction of cytoplasm with both nuclear genes and environment. Differences for critical photoperiod of TH and PH parents were also demonstrated as a probable source of reciprocal differences.