We report here an unusual activation of the Tc1 transposable element system in Caenorhabditis elegans. Germline Tc1 activity, as measured by reversion of unc-22::Tc1 alleles, is elevated 50- to 100-fold by certain crosses. For example, unc-22::Tc1 reversion is 1 x 10(-3) in a mut-6 IV strain and less than 1 x 10(-6) in a non-mutator strain, but in the unc-22::Tc1 progeny of a cross between mut-6 hermaphrodites and non-mutator males, reversion is 10(-1). The reciprocal cross does not induce this enhancement of reversion. Results similar to those for mut-6 were obtained using a mut-5 II strain. The mutator hermaphrodite by nonmutator male cross per se is not required for the enhancement of reversion, as mut-5 hermaphrodites x mut-6/+ males also induce unc-22 revertants at an elevated frequency. This reversion enhancement appears to depend on a maternal component inherited from a mutator strain, suggesting that the regulation of Tc1 activity may be complex.