SummaryMotility before and after cryopreservation of Acipenser ruthenus sperm was determined by computer-assisted sperm motion analysis (CASA). In addition, fertility of fresh and frozen/ thawed sperm was tested by in vitro fertilization. Sperm of A. ruthenus was successfully cryopreserved by 1/1 (v/v) dilution with ethylene glycol in final concentrations from 12.5% to 20%.Stepwise cooling using a programmable freezer and fast thawing in a water bath (40°C) for 3 s and 5 s, respectively, resulted in motility rates of 10% to 28% and fry yields of 44% to 94% compared to controls (fresh sperm). With fresh material, different sperm to egg ratios varying from 103 : I to lo6 : I gave similar fertilization rates of more than 90%. Using this cryopreservation method the establishment of sperm banks for endangered sturgeon species and the development of a fertility test for seasonally independent testing seem to be possible. IntroductionExperiments on cryopreservation of the sterlet (Acipenser ruthenus) sperm were carried out in order to examine the possibiliy to (a) establish a sperm bank of the Atlantic sturgeon (A. sturio) and (b) develop a new aquatic bioassay for seasonally independent fertility tests.The Atlantic sturgeon is considered as an extinct species in Germany as well as in many other European countries. Activities