Biodegradability (OECD Confirmatory Test involving active sludge) and toxicity (LC50 to a minute freshwater branchio-pod crustacean (Daphnia magna) and a small topminnow, viz., guppy (Lebistes reticulatus)) of ethoxylated 1-dodecanols was studied in relation to the M-distribution range of the ethoxylates and to the average degree (2-10) of polyaddition. Biodegradability, 86-91%, was evaluated as the arithmetic mean of 14 measurements (concentration drop, mg/L, of active substance responsive to Bi) carried out over 3 weeks. For Warsaw (1.6 min people, 74 m(3) wastewaters/s, assumed 1 kg ethoxylate used per capita), the LC50-data and the theoretical ethoxylate concentrations (0.01-0.002 mg/L) in wastewaters showed the narrow-distributed ethoxylates to produce a chemical load 5-10 times as great as that produced by the broad-distributed ethoxylates.