The heterozygosity of 13 polymorphic loci, encoding 10 enzymes from several pathways in carbohydrate metabolism, was tested for its effect on growth in juvenile manure worms Eisenia foetida (n = 169), raised under stressful (limited food, low moisture) followed by non-stressful (abundant food, high moisture) conditions. The predictive value of heterozygosity on growth was greatly improved by treating each locus separately rather than summing heterozygosity across all loci (multilocus heterozygosity). Under non-stressful conditions, heterozygosity among loci treated separately had no effect on growth rate (F = 1.60; df = 13, 155; NS); whereas under stressful conditions, heterozygosity among loci treated separately had significant and differential effects on growth rate (F = 234; df = 13, 155; P <0.01) with 7 loci contributing to a positive heterozygosity-growth rate correlation (r =0235; P <0.005) and 6 loci contributing to a negative heterozygosity-growth rate correlation (r = -0•202, P <0.01). Implicated loci could not be easily grouped into specific metabolic pathways. The negative contribution by some loci may arise as a consequence of heterozygote inferiority, linkage disequilibria with positively-contributing loci, or strong selection at that locus.