Experiments to determine whether rec-l, which increases allelic recombinationat the his-l locus in Neurospora crassa, also affects the am locus disclosed anothergene, rec-3. It appears that ree-l is specific to his-l and that ree-3 is specific to am, inthe sense that his-l is insensitive to rec-3 and am is insensitive to ree-l. The locus of rec-l is 18�9 units from the am his-l region; rec-3 is linked either to the am his-l region or to the mating type locus and 12�1 units from the relevant region.