Four 15-day old preruminant calves were fitted with two single cannulae, one in the lower ileum and the other in the upper colon. They were fed (on dry-matter basis) a milk substitute containing 35 p. 100 of dried, defatted, micronized alkane yeast (Candida tropicalis). Yeast carbohydrate digestion was studied in the ileum, colon and feces of two calves at 48, 72 and 102 days of age (three periods), using 1 p. 100 of chromic oxide in the diet as a marker. During each period, samples were collected every 2 hours and then pooled to obtain a sample from each part of the digestive tract. The carbohydrates were separated into 40 degrees C water-soluble (neutral and non-neutral) and water-insoluble fractions (fig. 1), and the composition of each was determined by hydrolysis and ion-exchange chromatography. The alkane yeast contained almost 22 p. 100 (in DM) of carbohydrate (table 1), mainly composed of 7.5 p. 100 of non-soluble mannans and 10.1 p. 100 of glucans, both from the hull, and 0.3 p. 100 of soluble galactans. The total carbohydrate content (table 2) was high in the ileum (23 to 28 p. 100 of DM) and low in the feces (3 to 8 p. 100 of DM). At the end of the small intestine, the soluble fraction contained some mannans and galactans partly bonded with non-carbohydrate compounds, but mainly neutral glucans (fig. 2). The apparent digestibilities (table 3, fig. 3) varied between animals and increased with age (total yeast carbohydrates: calf: 0.53 to 0.75; calf 2: 0.62 to 0.85). Mannans were better digested than glucans (calf 2: 0.95 and 0.57, respectively, at 102 days); the galactans were completely digested.