Environmental conditions such as nutrient availability may modify the strength and nature of interactions between mutualists. We examined the extent to which variability in nitrogen modified the interaction between ants and larvae of Glaucopsyche lygdamus Doubleday. Whereas nitrogen addition had no effect on larval growth rates, the number of larvae, or the number of ants tending larvae 2 wk after the beginning of the experiment, by 4 wk nitrogen addition had increased tending rates from 2.5 to 4.1 ants per larva. Despite the effect of nitrogen on tending rates of larvae, there was no effect of nitrogen on host plant growth, herbivory, or seed production. Unlike a previous study of this population of butterflies on a different host plant species, there was no indication that larvae feeding on pods attracted larger ant guards. Thus, although the amount of nitrogen available to plants did have a detectable effect on the ant–lycaenid mutualism, there was no evidence that plant substrate played a strong role in mediating tending rates. Because our finding that nitrogen addition affected tending rates parallels findings for a very different ant–lycaenid system, we suggest that nitrogen availability to host plants may be of general importance to ant–lyceanid interactions.