Internal parasites are a concern for industry as they negatively affect growth, survival, and reproduction. The objective was to evaluate the susceptibility of maternal lines to parasite infection, with the goal of developing genetic tests for this trait in US sheep. Purebred and Texel-sired lambs were born to Polypay, Katahdin, and Easycare (1/2 Romanov, 1/4 Katahdin, 1/4 White Dorper) ewes. Lambs were born on one of four 10-acre pastures, in a low-input system, 391 lambs were weaned at 70 d of age. Variation was accounted for by having all combinations of maternal lines and mating systems in each pasture. At weaning, a fecal sample was collected from individual lambs and the McMaster fecal egg count test (FEC) was performed on each sample. Data from 282 lambs were analyzed. For genetic evaluation, parentage was determined by allelic exclusion with genotype data from an international panel of 109 parentage SNPs. Data, log (FEC + 1), were analyzed using a mixed animal model with maternal line, mating system, maternal line × mating system, sex, and pasture as fixed effects. Variance components for additive genetic and environmental effects were estimated by REML using the iterative MIVQUE algorithm. Variance component estimates converged at positive values ensuring that the estimates were REML. Heritability was calculated as the additive genetic variance divided by the phenotypic variance after adjustment for fixed effects. The pasture effect for log (FEC + 1) was highly significant (P = 9.38 × 10^–16). Mean log (FEC + 1) of the 4 pastures were 2.66, 3.29, 3.90, and 2.93 ± 0.12, and the corresponding median FEC (back-transformed from log-normal distribution) were 454, 1944, 7927, and 845 eggs/g. However, breed cross and sex were not significant, with P values of 0.99 and 0.49, respectively. The heritability estimate was 0.28 ± 0.17 which is moderate and significantly different from 0 (P = 0.0025) based on the restricted likelihood ratio test. These results indicate that the fecal egg count phenotype for sheep raised in central Nebraska is affected by parasite exposure and host genetics.