The climatic condition in the region οf Manaus can be considered a special mi cro-region. The yield of soybean in different environments varies considerably. The objective of this study was to determine the best sowing date for soybeans at the Manaus region in "terra firme" soils. Ten sowing dates were used; the first being January 25 th, 1979, and others subsequently spaced of ten days intervals. A second experiment was made in the following agricultural year, starting at November 9th with subsequent sowing dates spaced of fifteen days intervals. The first experiment showed that grain yield was not significantly different in sowing dates until March 23 rd, after which time grain yield decreased. The second experiment indicated that sowing, from November 25 th until March 10 th produced statistically better results than other sowing dates. The best sowing dates for these two experiments are considered to be from January to the middlt of March.