An irregular pattern of transgene silencing was revealed in expression and inheritance studies conducted over multiple generations following transgene introduction by microprojectile bombardment of allohexaploid cultivated oat (Avena sativa L.). Expression of two transgenes, bar and uidA, delivered on the same plasmid was investigated in 23 transgenic oat lines. Twenty-one transgenic lines, each derived from an independently selected transformed tissue culture, showed expression of both bar and uidA while two lines expressed only bar. The relationship of the transgenic phenotypes to the presence of the transgenes in the study was determined using (1) phenotypic scoring combined with Southern blot analyses of progeny, (2) coexpression of the two transgenic phenotypes since the two transgenes always cosegregated, and (3) reactivation of a transgenic phenotype in self-pollinated progenies of transgenic plants that did not exhibit a transgenic phenotype. Transgene silencing was observed in 19 of the 23 transgenic lines and resulted in distorted segregation of transgenic phenotypes in 10 lines. Silencing and inheritance distortions were irregular and unpredictable. They were often reversible in a subsequent generation of self-pollinated progeny and abnormally segregating progenies were as likely to trace back to parents that exhibited normal segregation in a previous generation as to parents showing segregation distortions. Possible causes of the irregular patterns of transgene silencing are discussed.