Nitrogen (N) fixation can be a dominant source of N to low-N streams, but little is known about seasonal variability. Streams in the Andean Piedmont of Venezuela have high N-fixation rates that contribute a substantial amount to the N-budget during the dry season, which has low-flows and clear water from December to April. The wet season is characterized by high bed-scouring flows and turbid water from May to December, which may limit N-fixation. To test this hypothesis, we measured N-fixation rates in 4 streams using the acetylene reduction method. We sampled during the dry season, and early and late wet seasons. N-fixation rates were highest during the dry season (~10 mgN/day/m), when N demand by autotrophs was likely the highest. In contrast, N-fixation was low early in the wet season, but was negligible (i.e. 0 mgN/day/m2) by the end of the season. The variability within the wet season is most likely due to light limitation early in the season, whereas later in the season, substrate was often scoured by high flow events. Dry season N-fixation can contribute up to half of the NH4 uptake in these systems, whereas during the wet season, it appears N-fixation does not contribute greatly.