This part of ISO 5667 gives practical guidance on sampling, pretreatment, performance and evaluation of waters in the context of biotesting. Information is given on how to cope with the problems for biotesting arising from the nature of the water sample and the suitability of the test design. It is intended to convey practical experience concerning precautions to be taken by describing methods successfully proven to solve or to circumvent some of the experimental problems of biotesting of waters. Reference has been made as far as possible to existing International Standards and guidelines. Information taken from published papers or oral communication is utilized as well. Primarily dealt with are substance-related problems concerning sampling, pretreatment and preparation of water samples for biotesting and treatment of samples during the test, especially when performing tests with waters and waste waters containing unstable or removable ingredients. Basic principles of quality assurance, evaluation of data and presentation of results are outlined. Special emphasis is laid on ecotoxicological testing with organisms ('single-species biotests'). Some features addressed in this general guidance apply as well to biodegradation and/or bioaccumulation studies as far as sampling and sample preparations is concerned. Preparation of poorly soluble substances and testing beyond the water-solubility limit is also addressed. This part of ISO 5667 is not applicable to bacteriological examination of water. Appropriate methods are described in other International Standards.