Observations of the chlorofluorocarbon CFCl3 obtained several times daily over the period July 1978 to June 1981 at Adrigole, Ireland (52°N, 10°W); Ragged Point, Barbados (13°N, 59°W); Point Matatula, American Samoa (14°S, 171°W); and Cape Grim, Tasmania (41°S, 145°E) are reported. In addition, observations at Cape Meares, Oregon (45°N, 124°W) are given for the period January 1980 to June 1981. On January 1, 1980, the average mixing ratio of CFCl3 in the lower troposphere is estimated to have been 168 pptv, and this is calculated to have been increasing 5.7% annually. An optimal estimation procedure for deriving the atmospheric lifetime of CFCl3 by using a nine-box two-dimensional model of the atmosphere is described. In this procedure, model parameters are estimated based upon minimizing the differences between the temporal trends observed and those calculated in the two-dimensional model. Assuming that the only destruction of CFCl3 occurs in the stratosphere, the lifetime, on January 1, 1980, estimated by the trend technique is years; the lifetime estimated from the global inventory of CFCl3 is years. The maximum likelihood current lifetime estimate obtained by combining the estimates from both analysis techniques is 78 years.