AbstractAn investigation has been made of the resistance time and upper lethal temperature of ammocoetes of four species of lampreys provided with a substrate into which they could readily burrow. In general, ammocoetes burrowed after transfer from the acclimation to the experimental temperature baths and later came out of the substrate only in lethal temperatures. A relationship was observed between the resistance time and the time taken to emerge, with the resistance time increasing exponentially with decreasing experimental temperature. InIchthyomyzon fossor, landlockedPetromyzon marinus, Lampetra (Lethenteron) Lamotteniiand inLampetra (Lampetra) planerifrom two different times of the year, the incipient lethal levels over a two week experimental period for larvae acclimated to 15° G were respectively 30.5, 30, 29.5, 28.5 and 28° C. Values forP. marinusacclimated to 5 and 25° C were respectively 29.5 and 31° C, whereas inL. planerithey were 28 and 29° C in April/May and 27 and 29° C in July/August. Extrapolation of the results for the three acclimation temperatures yielded ultimate incipient lethal levels of 31.4° G inP. marinusand 29.2 and 29.4° C forL. planeriexamined in the spring and summer respe