An Aspergillus nidulans strain, auxotrophic for pyrimidine, was transformed to prototrophy by means of microprojectile bombardment. The transformation frequency was somewhat lower than conventional polyethyleneglycol-mediated transformation of protoplasts. However, the percentage of stable transformants was considerably higher with the biolistic approach. Typically, integrations of several copies of the plasmid introduced into chromosomal DNA were observed. The effect of several parameters, like the concentration of conidia, chamber pressure during bombardment and size of microprojectiles, on transformation frequencies were investigated and compared to previously published data on microprojectile bombardment of fungal conidia. Optimum results (6 transformants/mu g plasmid DNA) were obtained when 10(8) conidia were bombarded with a helium pressure of 5.5-8.3 MPa (800-1200 lb/in(2)). M5, M10 and M17 tungsten particles were equally efficient.