The development of rumen microfauna in 7 Merino lambs from birth to 3 months was investigated. In the first week after birth, the genus Entodinium appeared; in the second week Polyplastron multivesiculatum; in the third week Eremoplastron bovis and Enoploplastron triloricatum; in the 5th week Diploplastron affine and Isotricha prostoma; and in the 8th week Ophryoscolex tricoronatus and Dasytricha ruminantium. The average numbers of ciliates were 7 x 10(3).ml-1 in the second and third weeks, 6 x 10(3).ml-1 in the 4th week, and later they oscillated between 3 x 10(3) and 1 x 10(4).ml-1rumen liquor.