A New Rearing Technique of Phytophagous Mite Tetranychus urticae Koch (Acari : Tetranychida) and its Application in the Study of the Efficacy of some Acaricides on Potato [Solanum tuberosum L.). A 5 cm diameter leaf disc of potato or another host plant (or four on 2.5 cm diameter) was used in a Petri dish of 9 cm diameter for the rearing technique. This leaf disc, pierced in its centre, slides along a rustproof pin and floats on a 1 mm thick lamina of demineralized fresh water. Water is a "strong barrier" which confines the tetranychid mites on the leaf disc, even if this one does not corne from a host plant (tetranychid mites deprived of food). This rearing technique was used as a bioassay to test the effectiveness of acaricides (pyrimiphos-methyl, bromopropylate, fenpropathrin, dienochlor) on the developmental stages of Tetranychus urticae. The ovicidal activity against the eggs of one, three, seven days old (the eggs incubation duration being 8.1 ±0.15 days) was also studied. The results obtained show that bromopropylate, fenpropathrin and dienochlor have an ovicidal activity against the eggs of the different ages, but dienochlor has the highest efficiency (90 % mortality). Pyrimiphos-methyl is only active against the seven-day old eggs and bromopropylate has a high efficiency only on the one-day old eggs. Concerning the other developmental stages such as chrysalis (protochrysalis, deu-tochrysalis, teleiochrysalis) and mobile stages (larva, protonymph, deutonymph and adult female), pyrimiphos-methyl has de highest efficiency (90 % mortality) ; dienochlor also, except mobile stages. Bromopropylate has no activity against the chrysalis and mobile stages and fenpropathrin has a remarkable repulsive effect.