Birefringent microstructure fibers are shown to allow efficient generation of frequency-tunable anti-Stokes line emission as a result of nonlinear-optical spectral transformation of unamplified femtosecond Ti: sapphire laser pulses. Femtosecond pulses of 820-nm pump radiation polarized along the fast and slow axes of the elliptical core of the microstructure fiber generate intense blue-shifted lines centered at 490 and 510 nm, respectively, observed as bright blue and green emission at the output of a 10-cm microstructure fiber.