In March 2000, "TANTAN", the Autonomous Underwater Vehicle (AUV) which was designed in particular for lake ecological survey, was launched. Based on the same concept as Tantan, a dam lake survey AUV "KUNI" was constructed, and launched in February 2001. The third vehicle for dam lakes in Shikoku Region was launched in December 2001. And, the forth one is under construction and will be launched in March 2002. In this paper, specifications of these four sister AUVs are presented and compared. "TANTAN", whose major missions are observation in the vicinity of thermocline and survey of oxygen concentration near the bed, is currently deployed in Lake Biwa. It is anticipated that they report special features of ecology in lakes which cannot be observed by conventional ways.