This paper reports the defence responses of plant cells to the stress of infection by mycoplasma using an algae model of Chlorella vulgaris under attack by the Mollicute, Acholeplasma laidlawii, which is normally a pathogen of animal systems and higher plants. When the two unicellular organisms were mixed, there was a significant rise in the heat flow rate from 30 min after the chlorella cells were mixed with the mycoplasma cells and were thermally equilibrated in the instrument. There were two peaks in the formation of the superoxide anion radical (O2√−). The first one appeared immediately after mixing the two organisms and essentially was completed in 10 min. This coincided with the raised oxygen uptake rate (OUR) for the supply of the superoxide that can destroy the foreign organisms. The second transient “burst” of superoxide anion radical production was observed after 40–50 min, and correlated with both the continued high level of OUR and the observed “heat burst”. The increased OUR by the chlorella cells at the early stages of infection may be the trigger for further, more prolonged defence reactions by the cells treated with mycoplasma. The continued infection with mycoplasma, depressed the rate of growth and division of the chlorella cells. This might be caused by competition of the former for substrates required by the host cells. The heat flow rate was also reduced, reflecting the depressed metabolic activity of the plant cells after prolonged exposure to the pathogen.