A set of seven primary trisomics and seven derived types were produced in Avena strigosa Schreb., a diploid oat species. Each trisomic type had a distinct phenotype. The primary trisomics were identified on the basis of plant and chromosome morphology. Although fertility (seed-set) and the frequency of transmission of the extra chromosome varied among selfed progenies of the various trisomics, each trisomic can be maintained. The trisomics occurred at a higher frequency in small seed fractions than in samples of large seeds. No straightforward relationship was apparent between chromosome length and trivalent formation but some association between trivalent formation and transmission rate was observed.Several qualitative and quantitative traits were compared between the primary telotri-somics, between these and the disomic and between opposite arm telotrisomics and their corresponding primary trisomic. It was concluded that the results are more compatible with dosage effects than with the gene balance theory.