Pseudomonas aeruginosa was able to grow in high levels of an amphoteric and a quaternary ammonium compound following repeated subculturing in increasing concentrations of the biocides. Resistance was acquired and lost gradually. Adaptation to both biocides resulted in cross resistance to biguanides but whereas quaternary adapted cells were resistant to a range of quaternary ammonium compounds, the amphoteric adapted organisms were not. Amphoteric adapted cells had increased hydrophobicity and exhibited ultrastructural modifications which suggested that the outer membrane might be involved in resistance. Both amphoteric and quaternary ammonium adapted organisms showed changes in their fatty acid profiles consistent with outer membrane modification but the changes were different in each case. The mechanisms involved in biocide resistance are discussed.