The objective of this study was to test for effects of gametic imprinting on litter size in swine by estimating variances for parent-specific gametic effects. Data were 64,047 and 137,009 multiparous records of number born alive for the U.S. Landrace and Yorkshire breeds, respectively. The statistical model included fixed effects of parity number and herd, and random effects of herd-year-season, mate, permanent environment, animal (additive genetic), and either maternal or paternal gametes. A Bayesian approach that used Gibbs sampling to obtain posterior distributions was employed. To aid in the interpretation of results, the Landrace data structure was used to simulate data with and without effects of imprinting. Analyses of the simulated records indicated that the model applied was capable of detecting effects of imprinting when such effects were present. Small, but non-zero, estimates of gametic variances were obtained when no imprinting was simulated. Estimates of the proportion of total variance accounted for by paternally transmitted gametes were 0.8 and 0.9% for Landrace and Yorkshires, respectively. These estimates were different from zero, but were similar to the results observed for data simulated without an imprinting effect. Corresponding results for maternally transmitted gametes were 1.6% for Landrace and 0.8% for Yorkshires. The estimate for Landrace was significantly greater than that observed for Yorkshires and for the simulations without a true effect and suggested the presence of a non-Mendelian genetic influence on litter size. Paternally imprinted genes are a plausible reason for the observed results. Assuming that the effect observed was due to paternal imprinting at a single biallelic locus, the substitution effect of the superior allele could be greater than 0.7 piglets per litter. Identification of a genetic marker for such an allele would be useful in marker-assisted selection of females. Other possible explanations exist for the increased gametic variance in the Landrace breed, but these explanations (such as maternal or cytoplasmic effects) may be less likely than paternal imprinting.