Shallow subsurface dissolved methane maxima were commonly observed in the northwestern Gulf of Mexico during a series of cruises between 1975 and 1977. Although there were often several methane maxima at various depths at any one particular station, the most prevalent and widespread occurred over a narrow band of &sgr; in a range from 24 to 26 and could be traced as layers extending along and outward from the continental shelf. These layers generally followed the local stratification and were associated with the upper part of the pycnocline. Advection was undoubtedly of importance in determining the extent and distribution of these methane maxima layers, but in situ production appears to have supported them. Some vertical profiles revealed associations between methane, ATP, and suspended matter maxima. It is postulated that methane forms in situ in reducing microenvironments associated with suspended particulates, which are advected from the shelf or which have accumulated in the upper pycnocline due to increase in buoyancy forces.