Tetrahymena thermophila BF(5) produce heat by metabolism and movement. Using a TAM air isothermal microcalorimeter, the power-time curves of the metabolism of T. thermophila BF(5) during growth were obtained and the action on them by the addition of Cr(VI) were studied. The morphological change with Cr(VI) coexisted and biomass change during the process of T. thermophila BF(5) growth were studied by light microscope. Chromium has been regarded as an essential trace element for life. However, hexavalent chromium is a known carcinogen, mutagen, cytotoxicant and strong oxidizing agent. Cr(VI) of different concentration have different effects on T. thermophila BF(5) growth with the phenomenon of low dose stimulation (0-3×10(-5)molL(-1)) and high dose inhibition (3×10(-5) to 2.4×10(-4)molL(-1)). The relationship between the growth rate constant (k) and c is a typical U-shaped curve, which is a characteristic of hormesis. T. thermophila BF(5) cannot grow at all when the concentration of Cr(VI) is up to 2.4×10(-4)molL(-1). The microscopic observations agree well with the results obtained by means of microcalorimetry. And T. thermophila BF(5) had obviously morphological changes by the addition of Cr(VI).