Photodegradation is an important sink for highly toxic methylmercury (MeHg) in aquatic ecosystems. Lakes have been extensively studied for MeHg photodegradation, but much less is known about streams, mainly because of the heterogeneity in sunlight availability along stream reaches and because there has been a lack of tools with which to integrate this longitudinal variability. We utilize odd-mass anomalies of stable Hg isotopes (i.e., Δ199Hg) as a proxy for estimating the relative extent of MeHg photodegradation in streams. In a northern California stream network, levels of MeHg in water and biota increased with increasing stream size in headwater and intermediate streams (drainage areas ranging from 0.6 to 150 km2), but MeHg levels decreased substantially in larger streams (drainage areas up to 1212 km2). In smaller streams, the increase of MeHg levels is attributed to increasing in situ MeHg production and is accompanied by only a small increase in Δ199Hg, indicating that the rate of MeHg photodegradation is low relative to the rate of in situ MeHg production. In larger streams, decreasing MeHg levels coincided with significant increases in Δ199Hg of MeHg (an average increase of 1.5% 6 0.5%, n 5 4), indicating that increasing MeHg photodegradation reduced MeHg levels in these wider, more open channels. Our findings clearly indicate that increasing sunlight availability in stream channels substantially increases MeHg photodegradation, which can reduce MeHg contamination in stream food webs.