The publisher regrets that in the above-mentioned article Fig. 3 was printed wrong. The correct version is reprinted here. Electron micrographs of exponentially growing cells of Z. bailii. A,B: Untreated control cells. Notice nuclei (N) and mitochondria (M) with a normal fine structure. C–F: Cells treated with 320 mM acetic acid for 10 min. C,D: Some nuclei (N) show chromatin condensation and most mitochondria are altered (arrows), with swelling, reduction of number of cristae and with myelinic bodies. E,F: Details of altered mitochondria. Note in E chromatin condensation in the nucleus (N). A: ×22 000; B: ×15 800; C: ×17 000; D: ×15 800; E: ×38 000; F: ×38 000.