Results are reported on the fertilization (to 18%) of steelhead trout (Salmo gairdneri) eggs with cryo-preserved sperm. Fresh semen was progressively diluted to 1: 4 with an extender containing 5 or 12.5% dimethyl sulfoxide, sealed in 1.0-ml ampoules, frozen in the vapor of liquid nitrogen, and subsequently thawed in a 4 C water bath. The fertilized eggs and alevins produced appeared normal when compared with those from fresh spermatozoa.