Laboratory and field tests were made to investigate the acaricidal properties of Tedion (2,4,5,4′-tetrachlorodiphenyl sulphone) and Kelthane (l,l-di(p-chlorophenyl)-2,2,2-trichloroethanol) and to compare them with Akar 338 (ethyl 4,4′-dichlorobenzilate), malathion (in field tests only), lime sulphur and the organophosphorus compounds Rogor (O,O-dimethyl S-methylcarbamoyhmethyl phosphorodithioate) and Ekatin (O,O-dimethyl S-2-(ethylthio)ethyl phosphorodithioate) against the red spider mite, Oligonychus coffeae (Nietn.), on tea in Assam during 1958–60. The materials were available at the following concentrations of active ingredient: Tedion emulsifiable concentrate (8 per cent, w/v), Kelthane wettable powder (18·5 per cent, w/w), Akar 338 emulsifiable concentrate (25 per cent, w/v), malathion emulsifiable concentrate (56 per cent, w/v), lime sulphur solution (24–26 per cent, polysulphide), Rogor emulsifiable concentrate, (40 per cent, w/v) and Ekatin emulsifiable concentrate (20 per cent. w/v). Tedion was also available as a wettable powder (8 per cent, w/w) and Kelthane as an emulsifiable concentrate (18·5 per cent. w/v). In the laboratory, the first three and Rogor were usually diluted to 1 part in 500 with water, Ekatin to one part in 1,000 and lime sulphur to 1 in 40, but Tedion was also tested in some experiments at 1:750, 1:1,000 and 1:1,500. Using a single-leaf technique in the laboratory, 2 ml. of spray were applied to each leaf. Sprayed directly on to the eggs, Tedion and Kelthane gave equally good kills (97·7 and 90·7 per cent., respectively) and were more effective than Akar 338 (47·9%) or Ekatin (27%). There was some evidence that Tedion may be more effective as a deposit on which eggs are subsequently laid than as a direct spray. It also has a toxic effect on the eggs developing in the ovary, maximum effect (0·5 per cent, viability) being observed in eggs laid on unsprayed leaves on the second day after spraying of the female. The effect declined slowly towards the fifth day and by the ninth day had practically been lost. Rogor was not tested against the eggs. At the concentration effective against eggs, Tedion had only a limited effect (38·7 per cent, mortality) on nymphs and adults 24 hr. after application. However, in two laboratory trials carried out on potted plants to compare the toxicities of Tedion, Akar 338 and lime sulphur under these conditions and to ascertain the persistence of their effects against nymphs and adults, whereas Akar 338 and lime sulphur were significantly more effective than Tedion after 24 hr. at the concentrations used, after one week the effect of the two former showed little change, whilst that of Tedion had increased greatly, and after two weeks was significantly greater than that of Akar 338 in both trials and in one of them after four weeks. The effect of lime sulphur at four weeks was not significantly different from that of Tedion and was superior to that of Akar 338 in one trial. Kelthane, Akar 338, Eogor and Ekatin, and also lime sulphur, gave 100 per cent, kill of nymphs and adults 24 hr. after application by the single-leaf technique. Kelthane, Rogor and Ekatin were not tested for persistence of effect. In the field, all sprays were applied at 100 gal. per acre. The same dilutions of Tedion, Kelthane, Akar 338 and lime sulphur as were used in the laboratory were applied, giving rates of 2·6, 6, 8, and 100 oz., respectively, of active ingredient per acre. Malathion was diluted to 1 part in 500, giving 18 oz. active ingredient per acre, and Eogor and Ekatin to 1 part in 1,000, giving 6·4 and 3·2 oz. per acre, respectively. Counts of numbers of mites on leaf samples taken 3, 7 and 15 days and one month after application in two trials in 1959 and one in 1960 indicated that, at the concentrations used, Tedion, although at almost every count it had reduced the mite population by over 90 per cent., as compared with the control, was not consistently significantly superior to Akar 338, Kelthane and Rogor. There was some indication that Ekatin and malathion might be less effective than Akar 338, Kelthane and Eogor. Lime sulphur was effective for a period up to 7–15 days.