Mutant strains requiring lysine and arginine were produced by ultraviolet irradiation of zoospores of A. arbuscula. Analysis of crosses involving these mutant stocks reveal that the cross wild-type x lysine-less showed a 1:1 segregation for the lysine requirement while the cross wild-type x arginine-less showed an approx. 1:1 segregation for the arginine requirement. This anomalous segregation might result from the presence of a semi-lethal factor in the genom of this arginine-less strain. The cross arginine-less x lysine-less did not give a 1:1 segregation. This unexpected behavior has not been satisfactorily explained. The results of these crosses confirm previous genetic analysis of the life cycle of this organism.