Mixed colonies of adults of Lytta nuttalli Say and L. viridana Le Conte, and of L. cyanipennis (Le Conte) and nuttalli, and colonies containing only one of these three species have been seen in various parts of the Canadian prairies, usually in aggregations and most commonly near bodies of water. The flight period of nuttalli extends from early June to early August, of viridana from late May to late June, and of cyanipennis from mid-June to early July. The natural food plants of these three species are a variety of wild legumes, though they prefer certain cultivated legumes when present. Nuttalli and viridana will eat some non-leguminous plants, including species of Iris, Delphinium, and Brassica, and such crops as sugar beets, oats, barley, and wheat. In captivity, males and females of nuttalli and viridana mated more than once and females deposited more than one batch of eggs. Egg batches of nuttalli, viridana, and cyanipennis contained an average of 320, 340, and 390 eggs each, respectively. Cross-mating between nuttalli and viridana and nuttalli–viridana hybrids were observed in the field. Four nuttalli females were seen ovipositing in an area that contained the remains of nesting cells of leafcutter bees (Megachile sp.); these are the first recorded observations of Lytta species ovipositing in the field. The beetles preferred moist, firm soil for oviposition, and the eggs desiccated very quickly when not in contact with moisture. The eggs of these three species developed fairly uniformly at temperatures from 15° to 30 °C, and it appeared to take about 2 to 3 weeks to complete embryological development at temperatures normally encountered during June through August. Relationships among these three species suggested by their behaviour and biology are discussed.