A critical examination of the utility of protozoan bioassays in marine as well as in freshwater ecotoxicology implies the answer to three basic questions: why, how and when. Arguments in favor of tests with Protozoa are: protozoans combine all biological mechanisms and functions in one single cell; the physiology and biochemistry of several species has been studied extensively; the generation time of Protozoa is very short in comparison to Metazoa; large numbers of organisms can be produced in a small volume; induction of cryptobiotic stages is possible with some species; Protozoa play a significant role in aquatic ecosystems; test methodologies with very sensitive criteria can be worked out. The two basic approaches for bioassay are in situ tests and laboratory tests. In situ tests comprise e.g. the study of the modification of colonization rates on natural or artificial substrates in polluted areas, or the evolution of natural populations in artificial enclosures or cages to which a toxicant is added. For laboratory tests a multitude of methodological procedures based on different criteria can be worked out taking into consideration some of the basic advantages of unicellular species as compared to pluricellular organisms. Such laboratory tests are, however, highly dependent on the continuous availability of stock under routine culturing conditions. Controlled production of cryptobiotic stages can be extremely helpful in this regard. In the light of the foregoing the extreme scarcity of data on the effect of toxicants on marine Protozoa and the very small number of test protocols available is quite surprising. Ecological research has for a long time emphasized the role of protozoans in aquatic ecosystems, not the least in the transformation and degradation processes of organic matter; in this regard protozoan tests can be very helpful in determining the potential interference of xenobiotic chemicals with these activities and also in assessing the role of this group in accumulation, biodegradation and/or transfer routes of pollutants in aquatic ecosystems. Since techniques in culturing protozoans for fundamental research are well established and standard bioassay technologies have been developed with several species of unicellulars in pharmacology and medicine, simple routine tests can in principle, and in analogy, be worked out quite easily with Protozoa for marine hazard assessment. The development of such methods would be very welcome to complement the restricted number of standardized marine ecotoxicological tests available to date and especially to bridge the gap between microbial tests and bioassays with metazoan species. The experimental approach of the few bioassays with marine protozoans quoted in literature is critically reviewed and suggestions are made on the R & D which should be undertaken to improve this situation.