Recently, the incorporation of exotic germplasm from Avena sterilis L. into cultivated oats (A. sativa L.) has become a part of several oat improvement programs. To date, this research has involved only nuclear genes; yet, variation among cytoplasms in Avena may provide much opportunity for improving oats as an agricultural crop. The objectives of this study were (a) to determine if cytoplasmic effects occurred in reciprocal interspecific matings, (b) to assess whether backcrossing to the cultivated parent influenced cytoplasm effects, and (c) to determine if there were specific nuclear x cytoplasmic interactions;Sixty populations of 20 oat lines each, representing the BC(,0), BC(,1), and BC(,2) of the matings among five A. sterilis accessions and two A. sativa cultivars with reciprocals, were evaluated in a field-grown experiment for grain yield, straw yield, harvest index, heading date, plant height, unit straw weight, and vegetative growth rate. The data from this experiment were analyzed in such a way that the effects of nuclear genes, of cytoplasms, and of nuclear genes x cytoplasms could be assessed;A. sterilis nuclear effects on grain yield, straw yield, and plant height were large, even in the BC(,2), indicating much genetic variability was present in A. sterilis for these traits. The importance of the A. sativa parents to differences among matings increased with backcrossing to the A. sativa parent for all traits except plant height and grain yield. With random backcrossing, specific nicking ability of A. sterilis and A. sativa was lost, with the result that parental performance per se was a good indicator of the performance of a mating;Generally, the values for straw yield, plant height, unit straw weight, and vegetative growth rate were increased by A. sterilis cytoplasm, but these advantages decreased with backcrossing and were lost by BC(,2). Probably, this relationship with level of backcrossing resulted because interactions between A. sterilis cytoplasms and A. sterilis nuclear genes were responsible for the improved performance. Harvest index showed an advantage for A. sterilis cytoplasm in the BC(,1) and BC(,2). Grain yield and heading date were significantly increased by A. sterilis cytoplasm and these effects were not diminished by backcrossing. The effect of A. sterilis cytoplasm on heading date showed no mating interactions of importance, an indication of a direct cytoplasmic effect. The grain yield increase due to A. sterilis cytoplasm was most likely due to specific favorable interactions with A. sativa nuclear genomes;These results suggest that plant breeders might improve crop performance by making use of diverse cytoplasms. An additional benefit would be the added cytoplasmic diversity which could be a stabilizing influence on production.