NATURALLY occurring tumours are extremely rare in insects. Harker1 in her review reports that, except for those in Drosophila, only two have been described during the past 25 years. Tumours can, however, be induced in cockroaches by cutting the nerves2,3, by repeated implantation of suboesophageal ganglia which are out of phase4, and by ligature of the salivary ducts5. We have tried to induce tumours experimentally in the German cockroach, and discovered that naturally occurring tumours (those occurring without known causes) were very frequent in our colony of the insect. If the recurrent nerve, which innervates the anterior parts of the intestine and the salivary organs, was severed, there was no effect on tumour frequency in our experiments on Blattella germanica. Similar results were obtained in the American cockroach, Periplaneta americana, after severance of the recurrent nerve5. We were therefore able to treat all our material as a single unit. Fig. 1 shows diagrammatically the distribution and frequency of tumours in the salivary organs and the intestines of a total of 246 individuals of Blattella germanica. This shows that tumours were found mostly in the ectodermal parts of the intestinal tracts and especially in the rectal parts. Tumour frequency increased with the age of the host and 96 per cent of older males were tumorous. Females from the same age groups were less susceptible than the males.