Specific in vitro-generated insertion, replacement, and deletion mutations have been integrated near the chromosomal locus of am (NADP-specific glutamate dehydrogenase) of Neurospora crassa. Two approaches have been successful. One approach used am+-containing vectors capable of integrating at any site in the genome. This technique was used to introduce a specific 700 bp insertion near the am locus and to replace chromosomal sequences near am with plasmid DNA. Efficiency was low, however, and many transformants had to be screened to find the desired alterations among the ectopic insertions unless the incoming DNA had a large region of homology with the am region. A second approach increased the efficiency by using vectors containing a truncated am gene, so that prototrophs could arise only by homologous recombination. Overall transformation frequency was reduced relative to the first method, but a large fraction of the transformations involved specific alterations of the am region.