Measurements of atmospheric methane from 1989--1996 at Baring Head, New Zealand, and at Scott Base, Antarctica show a seasonal cycle in the mixing ratio with a peak to peak amplitude of 28 ppb. This is superposed on a trend varying between 16 ppb yr-1 and near zero. Δ13C values also show a seasonal cycle, with an amplitude of 0.1--0.3?, approximately 6 months out of phase with the mixing ratio cycle. A pronounced negative anomaly in Δ13C occurred in 1992 with annual average values dropping from -47.08? to -47.28?. From 1992 to 1996, average Δ13C values recovered slowly at an average rate of about 0.04? yr-1. The simultaneous changes in the mixing ratio growth rate and Δ13C together with the rapid drop and slow recovery in the latter provide a stringent test of possible causes. Although a combination of causes cannot be ruled out, decreased emissions from an isotopically heavy source such as biomass burning best meet the constraints of the data.¿ 1997 American Geophysical Union