Nine mutants of N. crassa which require arginlne as a nutrient but cannot use citrulline were obtained from various sources. Each segregates as though a single mutant gene is involved. Genetically, they fall into 2 classes, according to location in linkage group I or VII. All crosses between the group VH mutants gave many arg+ progeny. For the 1 pair of mutants which was studied in detail, origin of the arg+ by means other than crossing-over (or gene conversion) has been virtually eliminated. However, mapping studies place the 2 mutants only between 0 and six units apart. It is concluded that the high arg+ frequency is due to selection, and that the mutants might be pseudoalleles.