Microwaves of 2450 MHz have been used as a heating procedure in food technology, tissue processing for histological studies (1), in medicine (2) and as a sterilization agent for medical devices (3). Experiments were carried out exposing yeast cells of Candida albicans suspended in growth media to different times of radiation at 750 watts in a commercial microwave oven. Batches of 5 tubes containing the cells suspension were radiated from 15 seconds to 3 minutes and colony counts were done after exposure. Viability ceased totally after one minute radiation. Samples were obtained from unexposed controls and tubes radiated for 15, 30, 45 and 60 seconds, centrifuged and the pellets fixed in 1.5% potasium permanganate. After embbeding in Epon, thin sections were obtained and stained with uranyl acetate. Micrographs were taken with a Zeiss EM-109 electron microscope. Cells showed alteration of internal membranes at early times, and later changes indicative of cytoplasmic coagulation and organelle disorganization were seen. At 15 seconds mitochondrial matrix showed alterations followed by damage of its membranes.