The effects of a diet deficient in fat-soluble vitamins on infectivity, growth, and development of Echinostoma trivolvis in domestic chicks were studied. The vitamin-deficient diet (experimental) contained 21% protein, but lacked vitamins A, D, and E. The control diet was isocaloric, contained more than 18% protein, and a normal vitamin complement. There were no significant differences in chick infectivity or worm recovery at 7, 10, or 14 days postinfection (p.i.) from hosts on either diet. Likewise, there were no significant differences in chick weights at 7, 10, or 14 days p.i. in hosts maintained on either diet. However, worm weights and body and organ dimensions were significantly greater at 10 and 14 days p.i. in worms from control versus experimental hosts; worms from experimental hosts never became ovigerous. Our results support Beaver's speculation that E. trivolvis develop poorly in avian hosts lacking fat-soluble vitamins.