Nineteen sensitive cell lines of U. maydis were crossed with three killer strains and sample progenies were screened for killer segregation patterns. Crosses involving 11 lines gave killer frequencies ranging from 71%–100% of the progeny and 4:0 segregations in tetrads. Segregations in some crosses involving each of the remaining 8 lines gave killer frequencies from 0%–58% and mixed tetrads containing both non-killer and killer meiotic products. Many of the killers were unstable on further culture. Killer suppression showed varying degrees of specificity, appeared to be cytoplasmically determined for at least one strain, and was associated with possession of dsRNA in this strain and one other. No dsRNA was detected in two other suppressive strains. There was no evidence for segregation of nuclear maintainer genes for any of the killer determinants.