Modern chemical residues from water pollution (such as herbicides, antibiotics and even chemical substances of military use) are a challenge to survival of microalgal populations. Adaptation of three microalgal species – Pseudanabaena planctonica (Cyanophyceae), Dunaliella tertiolecta (Polyblepharidaceae, Chlorophyceae) and Scenedesmus sp. (Chlorophyceae) – to growth and survival in the presence of erythromycin, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and 2,4,6-trinitrotoluene was analysed as an experimental model. Growth of these species was restricted even by micromolar concentrations of such xenobiotics. When such contaminants are added to a dense microalgal culture, the cell density will be reduced after a few days due to the death of sensitive cells. However, after further incubation for several days, the culture will sometimes increase in density again due to the growth of a cell variant which is resistant to the contaminants. A Luria–Delbrück fluctuation analysis was carried out to distinguish between resistant cells arising from rare spontaneous mutation and resistant cells arising from other mechanisms of adaptation. In all cases, the contaminant-resistant cells arise randomly by rare spontaneous mutation during replication of cells prior to the addition of the contaminant (pre-selective mutations). Since wild-type microalgal genotypes are unable to survive in the presence of such contaminants, spontaneous pre-selective mutation (i.e. mutation from herbicide sensitivity to herbicide resistance) offers insights into the evolutionary capabilities of microalgal populations in contaminated environments. The rate of spontaneous mutation from sensitivity to resistance ranged from 2·0×10−6 mutants per cell division for erythromycin sensitive [xrarr ] erythromycinresistantP. planctonica to 8·2×10−6 mutants per cell division for TNTsensitive [xrarr ] TNTresistantScenedesmus sp. Since contaminant-resistant mutants have a diminished fitness, the resistant variants are maintained in the absence of the contaminants as the result of balance between new resistant cells arising from spontaneous mutation and resistant cells eliminated by natural selection. Thus recurrence of rare spontaneous pre-selective mutations ensures the survival of microalgal population in suddenly polluted environments.