The effect of 16-hr food deprivation on day 3 and again on day 5 of pregnancy on the fecundity of female water voles homozygous (a e /a e ) or heterozygous (A/a e ) for, an allele at the Agouti (A) locus, non agouti extreme (a e ) was studied. 63 A/a e females (mated to a e /a e males) produced 115 food-deprived and 115 control pregnancies, and 52 a e /a e females (mated to A/a e males) produced 55 food-deprived and 57 control pregnancies. Regardless of the experimental group, pregnant a e /a e females weighed less than A/a e females. The effect of food deprivation on fecundity depended on the Agouti-locus genotype of the female. In food-deprived A/a e females, fecundity was diminished due to fewer successful pregnancies (P < 0.001) and lower survival of the young (P < 0.05). In food-deprived a e /a e females, reproductive performance was not changed; a somewhat reduced rate of successful pregnancies was compensated for by significantly increased (P < 0.002) postnatal survival of the young. In progeny weaned from food-deprived mothers, the frequency of A/a e females was diminished. Resistance of a e /a e females to the negative effect of nutritional stress, and predominance of a e /a e young in progeny produced by food-deprived mothers, may favour the maintenance of polymorphism for the Agouti-locus in natural populations of the water vole.