The present culture-proven cases are contrary to the generally held view that infection with Mycobacterium marinum is habitually a superficial infection without potentially serious consequence. A wide spectrum of pathological lesions may be seen in the synovium and adjacent tissues in patients infected by M. marinum. Variations in the morphology of the inflammatory reaction occurs both between cases and, to a lesser extent, in different areas of individual cases. The inflammation ranges from the common non-specific diffuse form, to lesser areas of focal non-caseating lesions, to rarer focal caseating types of granulomatous reaction, and can include an acute inflammatory cell component. Fibrinous exudate on the synovial surface is a recurrent feature and is often the site harbouring most acid-fast bacilli. The variation of inflammatory reaction with time and the possible effects of local steroid injection are discussed.