Abstract. The rumen microbiota were studied in free-ranging semi-domestic reindeer in Finnish Lapland under the nutritional conditions obtaining at two different sampling times. Qualitative and quantitative investigations were made of the rumen ciliate fauna and quantitative investigations of the rumen bacterial flora. The volume coefficients for rumen ciliates obtained by Westerling (1970) and that for rumen bacteria obtained by Warner (1962) were used to obtain an indication of the volume of the rumen microbe mass in reindeer.The rumen samples were collected in connection with the round-up and slaughter of reindeer, being taken from 30 animals in December and 29 animals in March. The reindeer slaughtered in December had normal access to food,but those slaughtered in March had grazed on better pastures and received a supplementary feed of hay.The total number of ciliate cells was over six times as high in March as in December, the numbers being 1 182 900 and 188 300 per ml rumen contents, respectively. The corresponding total numbers of bacterial cells were 9.65 x 10 9 in March and 6.65 x 10 9 in December. The reason for the statistically significantly (P < 0.01) higher numbers in March than in December is probably the better nutritional conditions of the herd slaughtered in March, not the time of the year.The ciliate fauna consisted of 19 different species, although not all the species were found in every sample. The percentage composition of the ciliate fauna did not vary considerably between the two sampling times.The volume of the total microbe mass constituted 8.2% of the rumen contents in March and 1.9 % in December, the average being 5.1 %. The proportion of the ciliate volume in the total microbe mass was clearly higher than that of the bacteria at both sampling times: 7.2 times as high in March and 1.7 times in December, the average being 4.7 times.Since the importance of the rumen microbiota for the utilization of food by ruminants was established, many investigations have been made of the numbers and kinds of rumen microorganisms and the factors affecting them (Warner 1965, Hungate 1966. The majority of the studies have been devoted to the rumen microbiota of domestic animals, but some have also dealt with semi-domestic or wild animals (Giesecke 1970). An extensive review of the