Growth rates in shell height on the shore and the number of eggs released in the laboratory during the spawning season were extremely variable among Littorina neritoides from a population on Anglesey, N. Wales. Much of this variability was attributed to the effects of unpredictable weather conditions on feeding activity. Thirtyper cent of the individually tagged snails in the population did not grow during 1976–8. Growth increments of the remaining snails were used to estimate the growth parameter, k, of the von Bertalanffy growth equation, and the asymptotic size, h∞, used in the equation was estimated to be 7·5 mm, corresponding to the largest individual found in the population. The von Bertalanffy equation was then used to construct a growth schedule for L. neritoides from post-settlement to the attainment of the asymptotic size. From this schedule, snails were expected to take at least 9 months to grow to a shell height of 2 mm and at least 5 years to grow to 7 mm. This growth schedule agrees closely with growth rates observed by Lysaght (1941) in a population at Plymouth, but predicts much lower growth rates than those estimated by Daguzan (1976) for a population in Brittany. Eggs were released from March to June in 1977 and from January to June in 1978. The extreme individual variability in egg production largely obscured the underlying relationship between fecundity and body size. The maximum observed fecundity was for a 71 mm snail which released a total of 3866 eggs during the 1978 spawning season. The average fecundity was 22 eggs per day during the 1978 spawning season, or about 300 eggs per season.