Thiobacillus denitrificans was entrapped lire in agarose gel. The ability of entrapped cells to reduce nitrate was monitored by the activity of the enzyme nitrate reductase. High enzyme activity was obtained from the entrapped cells, and in general the enzyme acts similarly as it was in free cells. Entrapped cells were able to reduce nitrate to nitrite when nitrate solution was perfused through a tubular reactor containing cell-gel.