Methane and nonmethane hydrocarbon (NMHC) data were collected in the Atlantic marine boundary layer from 40 ¿N to 32 ¿S latitude during October and November 1980. Average volume mixing ratios for CH4 of 1.69¿0.02 ppmv were obtained north of the intertropical convergence zone (ITCZ) and 1.60¿0.02 ppmv south of the ITCZ. The hemispheric CH4 gradient was very sharp, decreasing rapidly between 14 ¿N and 10 ¿N latitude. Gaseous NMHC concentrations (based on response as CH4) were measured simultaneously and found to lie consistently within the standard error for the individual measurements (¿0.02 ppmv) after correction for water vapor interference. The correction for water vapor response was large, frequently as much as 80% of the raw NMHC signal. Application of the student's t distribution test to the NMHC data set established that the mean northern and southern hemispheric NMHC mixing ratios measured, 0.016 and 0.010 ppmv, respectively, were statistically different at the 95% confidence level. These results indicate volatile NMHC mixing ratios average less than 0.02 ppmv over the Atlantic and represent the first set of semicontinuous NMHC measurements spanning the North and South Atlantic marine boundary layer.