A field study has been made in the relatively narrow sympatric zone of Triturus cristatus and T. marmoratus, in the department Mayenne (France). Ecological data have been analysed. In the sympatric area specific preferences appear to exist: ecological isolation has developed in some areas. Typical T. marmoratus areas are hilly and wooded, provided with many terrestrial hiding-places, while forest pools and springs especially serve as spawning sites. T. cristatus areas are flat and open. Comparison with distribution data of 24 years ago shows expansion of T. cristatus at the expense of T. marmoratus, which presumably is true for the whole sympatric zone. The preferences of the species in combination with changes in environment by human intervention explain the detected changes in distribution. Ecological isolation as a phenomenon in the speciation of both species is discussed.