Arachnids are important predators in arthropod assemblages that occur from early successional old fields to mature forests. As generalist predators, they may occupy varying trophic niches in different environments that have different prey available. Given that trophic niche can be quantified by the enrichment of ¹⁵N in an organism, we tested the null hypothesis that the trophic niche is invariant between old fields and forests in three groups of arachnids: cursorial and web weaving spiders (Araneae), and harvestmen (Opiliones), by comparing the stable isotope ratios of nitrogen (δ¹⁵N) in specimens of each group collected from field and forest. Cursorial spiders fed at the highest trophic level in both habitats, and likely consumed other predators as well as herbivores in the field. Web weavers showed slightly higher δ¹⁵N in the field than in the forest. Harvestmen and cursorial spiders had significantly lower δ¹⁵N in the forest, indicating a trophic shift downward during later succession when they may increase the proportion of detritivore prey in their diets.