We present summaries of 703 previously unanalyzed ozonesonde launches at six stations distributed along a meridional band near 75¿W between 9¿ and 53¿N. Altitude and seasonal variations are examined to assist understanding the patterns and relative roles of injection, transport, loss, and synthesis of ozone in the middle troposphere. By comparison with other North American stations we conclude that a longitudinal gradient of 10--20% is likely, increasing eastward. The data are generally consistent with conventional understanding of tropospheric ozone in terms of increased injection from the stratosphere during spring at high latitudes, southward transport with molecular lifetimes of several months, and a principal sink at the ground, but details of the altitude and seasonal behavior can be interpreted ambiguously as evidence either for shorter-scale transport or for ozone synthesis within the troposphere.