The electromagnetic proton cyclotron anisotropy instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi-Maxwellian with T⊥p/T∥p>1, where ⊥ and ∥ denote directions relative to the background magnetic field Bo. Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropies of the form T⊥p/T∥p-1=Sp/&bgr;∥p&agr;p where &bgr;∥p≡8&pgr;npkBT∥p/Bo2, and the fitting parameters Sp≲1 and &agr;p≃0.4. Plasma and magnetic field observations from the ACE spacecraft reported here show for the first time that this constraint is statistically satisfied in the high speed solar wind near 1 AU, and magnetic power spectra provide evidence that this instability is the source of the constraint. ¿ 2001 American Geophysical Union