Abstract 1. 1. The reported diversity of excretory products of spiders and the small number of species investigated in this regard prompted a reinvestigation of the excreta of these arthropods. 2. 2. Guanine was the only purine detected in the chromatographic analysis of excreta obtained from 34 species representing 17 families. Amino acids and urea were not detected in the chromatographic analysis of excreta of 4 species. Nesslerization of the excreta of 3 species did not reveal the presence of ammonium salts. 3. 3. Spectral analysis of individual extracts of excreta from 10 species confirmed chromatographic evidence and indicated that guanine made up 34–76 per cent of the excreta by weight. 4. 4. Kjeldhal determinations of these extracts revealed that guanine accounted for 73–87 per cent of the excreted nitrogen. 5. 5. The lack of certain enzymes in spiders was offered as a possible reason to explain the difference between uric acid excretion in insects and guanine excretion in spiders.