An HSR in chromosome 1 which is found in many feral populations of Mus musculus domesticus was shown in previous studies to consist of a high-copy long-range repeat cluster. One such cluster, MUT, showed distorted transmission ratios when introduced by female parents. MUT/+ offspring were preferentially recovered at the expense of +/+ embryos in the progeny of male MUT/+ x female +/+ but were found at the expected 1:1 ratio in reciprocal crosses. Preferential recovery of maternal MUT was due to lethality of postimplantation +/+ embryos. There was no distortion of the recovery ratio in MUT/+ x MUT/MUT progeny: maternal MUT and + clusters were present among live implants at a 1:1 ratio. Maternal and zygotic effects therefore contribute to the phenomenon. The mechanism of their interaction is unknown.