Nasal and tracheal swabs sequentially collected from three groups of eight calves between the ages of 1 and 98 days indicated that the nose and trachea were colonized by Mycoplasma spp. during the first weeks of life. Over 92% of all calves harboured Mycoplasma spp. in their noses when they were 2 weeks old, the rate of recovery falling gradually thereafter. The peak period of recovering mycoplasmas from the noses and tracheas of calves was at 6 weeks old. M. bovirhinis, M. arginini and Acholeplasma laidlawii predominated in the nose while M. dispar and M. bovirhinis predominated in the trachea. There was no association between rates of isolation and clinical signs of respiratory disease. There were no significant differences between the frequencies of isolation of Mycoplasma spp. from groups of calves kept under different environmental temperatures and relative humidities.