Birgus latro excretes nitrogenous waste as a mixture of urate and guanine and not predominantly urate as believed previously. The presence of guanine in faeces was confirmed by enzymatic derivatisation of guanine to xanthine with guanase. This is the first report of significant excretion of guanine outside the Chelicerata. The ratios of urate to guanine within the excreta of animals in field situations (natural diets) and in the laboratory (a range of artificial diets) were ∼3:2. Rates of excretion of both urate and guanine increased when experimental crabs were fed an artificial diet high in nitrogen. Significant amounts of guanine were also measured in tissues of B. latro, but only urate was present in equivalent tissues of the closely related species Coenobita brevimanus. Coenobita brevimanus did not excrete any significant amount of purines with the faeces.