The reproductive ecology of two snakes, female Vipera aspis (terrestrial) and Elaphe longissima (semi-arboreal), was compared. Mean clutch sizes were close in the two species; 6.17 ± 2.50 (n = 69) in the asp viper, and 6.59 ± 1.38 (n = 29) in the Aesculapian snake. When controlled for body size, body condition (hence amounts of body reserves), measured at the beginning of vitellogenesis, correlated positively with litter size in the asp viper but with clutch size in the Aesculapian snake. As in most species, maternal body length positively influenced clutch or litter size. Thus, the trade-off between maternal reserves and growth may favour reserves in the asp viper, and growth in the Aesculapian snake. The asp viper is a bi- or triennial breeder (33% of reproductive-females each year), the Ausculapian snake is an annual breeder (77% of reproductive females each year). These differences may be related to their contrasting foraging ecology. The asp viper is a terrestrial ambush predator with a specialized diet (98% Microtus) based on un-predictable prey availability. This species moves slowly (9.07 ± 8.40 m/day during the active season), and has a small home range (3,024 m2); an increasing body mass (large body reserves) should not affect its activity abilities. The Aesculapian snake is a semi-arboreal predator, which feeds on a large range of prey including birds and eggs, and which is often active (118, 11 ± 134,55 m/day during the active season). This species has a large home range (11,400 m2); an increasing body mass (large body reserves) should be a handicap during arboreal displacements. Vitellogenesis depends on body reserves in the asp viper, while it depends on foraging success in the Aesculapian snake.