Abstract Spawning interactions among fertile male, female, and sterilized male sea lampreys Petromyzon marinus were examined by placing externally attached radiotransmitters on 4% (52 animals) of the population of sea lampreys that was introduced above a barrier to their passage in the Carp River, which flows into Lake Superior, during 1996 and 1997. Movements and interactions on nests made by sea lampreys were monitored using portable receivers for up to 20 d from the start of nest construction. Four times as many nests from the same number of introduced animals were found in 1996 than in 1997. Two sea lampreys with radio transmitters emigrated from the stream in 1996, and 13 animals emigrated or were missing from the stream in 1997 when a freshet occurred just before spawning. Only three male sea lampreys with transmitters were observed spawning, and these males visited more than one nest. We continuously checked four or more nests each year and observed that each nest was built, altered, or destroyed s...