The objective of this study was to develop rearing conditions and a successful longterm culture medium for the maintenance of Lemna gibba L. G-3, Lemna minor L. clone 6591 (California), 7102 (= LMS) (Kansas), 7101 (LMY) (Connecticut), and 7136 (46) (Illinois). A variation of Hoagland's medium was employed. Growth of L. gibba G-3 was found to become enhanced by additions of selenium, (4.2 μgL-1, Na2SeO4 = 100 μgL-1), tin (456.6 μgL-1, SnCl4 = 10 mgL-1), cobalt [20.3 μgL-1, Co(No3)2 6 H2O = 1 mgL-1], and vanadium (25.6 μgL-1, VOSO4 2 H2O = 1 mgL-1). Vanadium failed to elicit any increase in growth on addition to Hoagland's medium from L. minor 7102, 7101, or 7136. There was no response shown by L. minor 6591, 7136, and 7101 to Co nor by L. minor 6591 and 7136 to Sn. The 7136 clone failed to respond to addition of Se. Root elongation, biomass (dry weight at 60°C), number of fronds, number of plants, Kjeldahl nitrogen, and percent chlorophyll a and b are among the endpoints examined. A comparison of these endpoints is presented. All biomass endpoints (growth) are highly related, and dry weight should be used as a final endpoint.