Information on diseases of guinea-pigs is generally available in research institutes and breeding establishments. When dealing with an individual patient it is tempting to extrapolate from this pool of information, without realising that a number of factors can affect the disease situation in a colony which need not apply to the individual animal kept as a pet--or vice versa. Thus diseases may appear as an explosive outbreak in a group of cavies kept under uniform conditions while a single animal may be exposed to amateur management (Townsend 1975) and suffer in consequence. The diseases generally encountered in this species are dealt with here as they appear clinically.