To investigate upstream spawning migration behaviour of fertile and sterilized sealampreys (Petromyzon marinus), we attached radio transmitters to 149 sea lampreys, releasedthem in three tributaries of Lake Superior between 1993 and 1996, and followed their movementsfor up to 60 d. Emigration from spawning streams was variable between years and streams (10û50% of releases within a stream), appeared to be affected by the length of unimpeded stream,and was not affected by the sterilization procedure. Most sea lampreys that stayed in the streamsmoved upstream (64%), the majority moving between 2200 and 0300 hours. Total distancetraveled, daily movement rates, and habitat selected by sea lampreys did not differ betweenfertile males and sterile males. In all streams, sea lampreys selected habitat with significantlyreduced light levels and made use of habitat providing refugiaùbrush piles, overhanging banks,and boulders. Differences in emigration rates among streams implies that stocking rates of sterilemale sea lampreys may have to be adjusted by stream to achieve expected suppression of larvalsea lamprey production.