Strains ofAspergillus nidulans carrying thesltA1 mutation, conferring sensitivity to KCl and NaCl, also showed an arginine-sensitive phenotype whereby concentrations of thel-amino acid at or above 10 mM were toxic to growth. Sexual progeny of a cross between asltA1 mutant and a wild-type strain showed a co-segregation of salt and arginine sensitivity. Similarly, revertants to salt tolerance showed a loss of arginine sensitivity as didsltA1 strains that were transformed with a cosmid carrying the putativesltA1+ wild-type allele. In addition, arginine sensitivity could be relieved byl-ornithine. It is suggested thatsltA1 is a salt-sensitive allele of the arginase gene (agaA).