It has been shown that Legionella pneumophila proliferates intracellularly in more than ten species of protozoa, but the fate of the bacteria in Tetrahymena thermophila has not been reported. We investigated the multiplication of L. pneumophila Philadelphia-1 strain in micronucleated T. thermophila, and the effects of temperature and numbers of the bacteria ingested by the protozoa after in vitro feeding were studied. T. thermophila preyed actively upon the bacteria. After being ingested, the fate of the bacteria was affected by both temperature and the number of bacteria ingested. When the number of ingested bacteria was 30 per protozoon, the bacteria proliferated intracellularly at 35℃. The bacteria, however, could not proliferate at 28℃ or 32℃ though they survived in the protozoa. When the ingested bacteria was 10 per protozoon, the bacteria were killed in the protozoa at all of the temperatures tested. Electron microscopic examination revealed that the protozoa ingesting the bacteria remarkably swelled and that protozoan food vacuoles which contained L. pneumophila were studded with ribosomes.