Abstract.1Juveniles of the Australian lycaenid butterfly,Jalmenus evagorus(Donovan), secrete to ants a solution of sugars and amino acids, primarily serine. The attendant ants protect the larvae and pupae from parasites and predators.2The effect of caterpillar nutrition on the defence provided by ants was investigated. Potted food plants ofAcacia decurrenswere either given water containing nitrogenous fertilizer or were given water alone. Fertilized plants had a higher nitrogen content than unfertilized plants.3Fifth instar larvae ofJ.evagorasfeeding on fertilized plants attracted a larger ant guard than those feeding on unfertilized plants. In the absence of caterpillars, ants were not differentially attracted to fertilized and unfertilized plants.4In the presence of ants, over a 10‐day period, larvae on fertilized plants survived better than larvae on unfertilized plants. In the absence of ants larvae survived equally on fertilized and unfertilized plants. It is concluded that larvae on fertilized plants attracted a larger ant guard, and thereby survived better, than larvae on unfertilized plants.5Adult females ofJ. evagoraspreferred to lay egg batches on fertiized, rather than unfertilized plants, but they did not lay larger egg batche