Particle friction angles ϕ represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows. We determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force Fd required to pivot or slide a particle out of its natural resting place. Within each site, median ϕ values were very similar to previously reported relations, yet different enough between sites that a location-general predictive empirical relation would produce errors in ϕ of ¿10 degrees for Di/Ks>1. Furthermore, within a Di/Ks class at a given site the range in ϕ was as large as 80¿, much greater than the range of median values between classes for natural sediment mixtures. Using estimates of &tgr;c* from extensive bed load measurements made by Andrews and Erman <1986> at Sagehen Creek and the in situ ϕ measurements made in this study together with a theoretical model developed by Wiberg and Smith <1987>, we show that ϕ measurements made with this new method can be used to accurately predict &tgr;c* for natural, water-worked sediments. Additionally, these results confirm that a ϕ value ≪ϕ50 is more appropriate for predicting &tgr;c* of a given size class. ¿ 1998 American Geophysical Union