Bacteria are essential for the functioning of the mangrove ecosystem. The objective of this study was to examine the seasonal behavior of densities and cellulolytic activity of heterotrophic bacteria in black mangrove Avicennia germinans sediments and their relationship with litterfall, water, and sediment variables in a semi-arid subtropical coastal lagoon. The study was conducted in the coastal lagoon El Soldado, located in the central region of the Gulf of California's eastern coast. Water, sediment, and litterfall of Avicennia germinans were sampled during an annual cycle. In the water, physicochemical parameters and dissolved inorganic nutrients were determined; in the sediments, texture, organic matter, dissolved inorganic nutrients, as well as densities and cellulolytic activity of heterotrophic bacteria, were evaluated. The results showed higher values of densities and cellulolytic activity of heterotrophic bacteria in spring (4.9 log CFU g(-1); 75%) and summer (4.9 log CFU g(-1); 94%), and lower values in autumn (3.9 log CFU g(-1); 27%) and winter (3.9 log CFU g(-1); 11%). The highest water temperature values, ammonium in water, organic matter in sediments and mangrove litterfall were observed in spring and summer, showing the highest bacterial density. It is concluded that the bacterial densities and cellulolytic activity in black mangrove sediments from an arid region exhibit a seasonal pattern coupled with organic matter inputs and water temperature changes. This evidence is important because it contributes to understanding the role of bacteria in ecosystem functioning.