Following chronic retinol (vitamin A) deprivation leading to exhaustion of liver vitamin A reserves below 50 I.U. per liver hamsters were fed diets either deficient in ("Rd":250 I.U.A./kg in experiment I, 1000 I.U.A/kg in experiment II) or enriched with retinol ("Rw":10000 I.U.A/kg in experiment I and II). After 4 weeks some of the animals (36 in experiment I, 30 in II) were infected with 150 3rd-stage larvae of D. viteae, while clean animals were kept as controls. The retinol status, the immune response (indirect fluorescent antibody test: IFAT) and parasitological parameters were examined up to 8 (experiment I) and 12 weeks (experiment II) post infection (p.i.). Rd hamsters had levelling off of weight gain or weight loss, severely deficient retinol levels in serum and liver, and high mortality. Weight gain was less in infected than in uninfected hamsters, and the capacity of infected Rw animals to restore liver retinol was significantly lower than that of uninfected Rw animals. IFAT titres were similar in Rd and in Rw animals, but microfilaraemia was significantly enhanced at 8 and 10.5 weeks p.i. in Rd hamsters. While the number of worms recovered from Rd and Rw hamsters was similar, there was a significant increase in the ratio of female to male worms in Rd hamsters. Rd hamsters in experiment I produced 3.3 times the worm mass per 100 g body-weight than Rw hamsters. Also, the average mass per female worm was significantly higher in Rd than Rw in hamsters, and this parameter was negatively correlated with the liver retinol concentration in experiment I(r = -0.89). Retinol deficiency has a marked effect on growth and fertility of D. viteae in hamsters.