Summary1. We examined the influence of hydrologic seasonality on temporal variation of planktonic bacterial production (BP) in relatively undisturbed lowland rivers of the middle Orinoco basin, Venezuela. We sampled two clearwater and two blackwater rivers over 2 years for dissolved organic carbon (DOC), chlorophyll, phosphorus and bacterial abundance to determine their relationship to temporal variation in BP.2. Dissolved organic carbon concentration was greater in blackwater (543–664 μm) than in clearwater rivers (184–240 μm), and was generally higher during periods of rising and high water compared with low water. Chlorophyll concentration peaked (3 μg L−1) during the first year of study when discharge was lowest, particularly in blackwater rivers. Soluble reactive phosphorus (SRP) was very low in the study rivers (<3.8 μg L−1) and concentration increased during low water.3. Average BP was higher in clearwater (0.20–0.26 μg C L−1 h−1) than in blackwater rivers (0.14–0.17 μg C L−1 h−1), although mean bacterial abundance was similar among rivers (0.6–0.8 × 106 cells mL−1).4. Periods of higher chlorophyll a concentration (low water) or flushing of terrestrial organic material (rising water) were accompanied by higher BP, while low BP was observed during the period of high water.5. Interannual variation in BP was influenced by variations in discharge related to El Niño Southern Oscillation events.6. Seasonal variation in BP in the study rivers and other tropical systems was relatively small compared with seasonal variation in temperate rivers and lakes. In addition to the low seasonal variation of temperature in the tropics, low overall human disturbance could result in less variation in the inputs of nutrients and carbon to the study rivers compared with more disturbed temperate systems.