Thirty-three argB- to argB+ transformants of Aspergillus nidulans have been subjected to genetic and molecular analysis. Two showed high levels of mitotic instability although it is suggested that this is a consequence of heterokaryosis rather than instability of the transformation event. Most transformants resulted from the integration of the transforming DNA in tandem with the chromosomal argB locus. The maximum number of inserted sequences was two, to generate three copies of the argB locus. The other main transformant type showed replacement of the argB- mutation by the wild-type allele present on the transforming plasmid. Transformants were also recovered in which the transforming DNA had integrated into non-homologous chromosomal regions. Selfed or hybrid cleistothetica from all transformants, except the gene replacement types gave arginine requiring recombinants. Most transformants showed low levels of meiotic instability. Others displayed varying levels which in some cases differed between selfed and hybrid cleistotheticia. There was some correlation between meiotic instability and the nature of the transformation event. Diploid parasexual and aneuploid analysis located the integrated DNA in each transformant to chromosome III. Two transformants were isolated as heterozygous diploids. A third diploid was isolated as a stable mitotic segregant from one of the mitotically unstable transformants.