The effects of changes in host dietary carbohydrate and roughage on previously adult Hymenolepis diminuta were examined. In one experiment rat hosts were infected with five cysticercoids each, maintained on Purina Laboratory Chow (pellet ration) for from 21 to 28 days, then divided into treatment groups as follows: (1) continued feeding pellet ration, (2) fed a diet containing 30% starch as carbohydrate component, (3) fed a diet containing 30% sucrose, and (4) fed a carbohydrate-free diet. Worms from hosts on sucrose and carbohydrate-free diets rapidly decrease in weight, numbers of gravid and mature proglottids, and carbohydrate concentration. Lipid accumulates to high concentrations, and nitrogen rises slightly, but after a week nitrogen drops back to levels in the pellet and starch groups. Worms from rats on sucrose and carbohydrate-free diets have defective female reproductive systems, apparent from histopathology in late mature and gravid proglottids. In other experiments rats were infected and maintained as above, then changed from pellet ration to diets containing 30% cellulose, 5% cellulose, or no roughage. After 6 to 7 days worms from the high roughage diets are smaller, have lower lipid concentrations, and have slightly higher carbohydrate concentrations than worms from hosts on pellet rations, low roughage or roughage-free diets. Possible explanations for the observed effects are discussed in the light of pertinent knowledge of host intestinal physiology. Diet and feeding habits of the host may have a profound effect on its tapeworm parasites. Levine (1938) reported that limitation of host food decreased proglottisation in Davainea proglottina. Starvation for only 24 hr depressed proglottid production for a week following. Reid (1942) found that the scolex of Raillietina cesticillus shed its strobila after 24 hr of host starvation, a new strobila being formed only if normal feeding was resumed. The destrobilation of R. cesticillhs was correlated with a dramatic decline in the glycogen content of the worm. Evidence that tapeworms were affected by the carbohydrates in their host's diet was reported by Chandler (1943), who found that absence or restriction of carbohydrate in host diet resulted in reduced establishment of Hymenolepis diminuta and stunting. Chandler et al. (1950) observed similar effects of carbohydrate lack whether the worms were previously established or the rats infected after they were already being fed low carbohydrate diets. Further, they found that neither sucrose nor glucose were adequate substitutes for starch in the host diet, but worms from rats on diets containing glucose were larger than those whose hosts were fed Received for publication 19 September 1966. * This investigation was supported by Grants AI06153 and 5 TI AI-226 from the NIH, U. S. Public Health Service. diets with sucrose as the only carbohydrate. They attributed the difference between glucose and starch diets to the fact that glucose is absorbed much more rapidly from the host intestine, while the gradual digestion of starch would make glucose available over a greater length of host gut. These results were confirmed and extended by Read and Rothman (1957b), who also stated that the effect of carbohydrate deficiency was the same whether the worms were previously established or the infection was initial. It is necessary to assume, however, that the worms must have arrived at their stunted condition by two different routes. If the host is being fed a diet inadequate in carbohydrate when it is infected, then normal worm growth does not occur, and maximum size cannot be attained. In contrast, if the worm is already at maximum size when the host is changed to a low carbohydrate diet, then worm size must be reduced. Such reduction might be the result of destrobilation, as found by Reid (1942), or proglottisation rate might be decreased, as apparently is the case in D. proglottina (Levine, 1938). Effects of changes in host diet on previously established worms may have some importance in nature, helping to account for seasonal fluctuations in egg production. Oliger (1950) reported that cestodes of tetraonid birds destrobilated when the birds changed to their