The host-alternating life cycle ofMyzus lythri, an aphid of European origin that feeds on purple loosestrife (Lythrum salicaria), is detailed. The complex life cycle and low availability of primary host plants are hypothesized as limiting early-season development of aphid populations on purple loosestrife. Experiments were undertaken in 1993 to determine if large populations of this aphid could influence the growth and development of purple loosestrife. Two-year plants each inoculated with five aphids showed significantly lower root and shoot weight after a 7-wk period as compared with matched controls. Over half of 20 seedlings (3 mo old) inoculated with two or fiveM. lythriwere dead or clearly dying 11 wk after inoculation. Mass rearing for early-spring release and planting primary hosts are possible methods to augment the potential of this aphid to function as a biological control agent for purple loosestrife.