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This paper presents an overview of observational and fish sampling tech­niques for investigating fish lesions, morbidity and mortality. These sam­pling techniques and investigations are much like detective work and require attention to detail, common sense, technical proficiency and experience. To solve the mystery of a fish kill, the investigator must use available evidence and clues to piece together a series of events that often have long since passed. The cause of these field events may be chemical, biological or physical; more often, it is some combination of these. An initial categorization approach may be used to reduce the great number of possible causes of a fish kill to something more reasonable. Through proper observations, the most probable cause may be placed in one of four broad categories (although additional secondary relationships should also be recognized). These broad categories include oxygen related, toxics or water quality related, disease or population related and trauma related events, and may be based on defined criteria Caution should be taken on making etiologic generaliz.ations since many types of lesions or mortality events may appear similar. This paper provides support for making consistent observations; taking photographs, tissue and water samples; classifying external lesions and choosing appropriate ne­cropsy methods. A bibliography is provided to reference information perti­nent to fish kill investigations and fish disease, anatomy and taxonomy.