The life history strategy of the veined rapa whelk Rapana venosa, a temperate marine gastropod, includes generation times of 1 y, individual longevity of >10 y, annual production of egg cases, and an adult size range of 40 to >160 mm shell length (SL). This life history combined with the animal's generalist ecological preferences and broad physiological tolerances makes rapa whelks well suited for ecological success along a gradient of habitat and community types. Ballast water transport of veliger larvae across traditional zoogeographic boundaries has resulted in the establishment of invasive rapa whelk populations in Chesapeake Bay, USA, as well as European habitats. Although the need for a quantitative understanding of rapa whelk reproductive biology in terms of annual embryo production or fecundity is acknowledged as a necessary step in assessing the biological and ecological consequences of such an invasion, rapa whelk fecundity in relation to size for the entire adult size range (40–>160 mm SL) has not previously been quantified. The relationships between rapa whelk female size and egg case height, number of embryos egg case⁻¹, number of egg cases produced female whelk⁻¹ yr⁻¹ and fecundity, and the number of embryos female⁻¹ yr⁻¹, are described for a size range of individuals from Chesapeake Bay. In 2001 and 2003, egg laying began in mid to late May at water temperatures of approximately 18°C and salinities 14–20 ppt and continued for 11–12 wk. The smallest rapa whelk observed (45 mm SL, 5.8 g wet tissue weight) laid egg cases with heights of approximately 7 mm. The largest whelk (163 mm SL, 220 g) laid egg cases with heights 30–33 mm. The number of embryos observed in egg cases produced by Chesapeake Bay rapa whelks ranged from 123 embryos in a 7.4 mm high egg case to 3,673 embryos in a 33.5 mm high egg case. Rapa whelk fecundity (number of embryos female⁻¹yr⁻¹) increased with female size. Egg case production was observed in rapa whelks that had hatched from egg cases and recruited within the previous year (<1 y old). Small (<80 mm SL, <28 g wet tissue weight) rapa whelks have relatively lower annual fecundities (1 × 10⁵ embryos female⁻¹ yr⁻¹) than large (>80 mm SL, >28 g wet tissue weight) rapa whelks that may produce between 1 × 10⁶ and 4 × 10⁶ embryos female⁻¹ yr⁻¹. Estimates of rapa whelk fecundity, even for small rapa whelks, are at least an order of magnitude greater than similar estimates for native United States Atlantic coast gastropods.