Intestinal tract infection by Chlamydia psittaci was demonstrated in one cattle herd by isolation from faecal specimens, using embryonated eggs. Such infections were observed in all animals younger than 12 months, in 60% of the heifers and in none of the adult cows. The presence of infection correlated (r=0.511) with the serum titre of compliment fixation antibodies against chlamydial antigen. Young calves, which were spontaneously infected with Chlamydia postnatally, developed ileitis and moderate interstitial pneumonia. The results of histological sections and isolation of the agent from tissue specimens indicated Chlamydia to be the cause of these conditions. The strain isolated (ROS DK/KVL 6/B3) was identified as C. psittaci. The morphology of the organism and it's pathogenicity in guinea-pigs were studied. In embryonated eggs, a dose-response curve was demonstrated for the ROS strain, which differed in that respect from another member of this species tested, viz. EBA (59-795).