We combine data from the extremely deep Hubble Space Telescope U (F300W) image, obtained using WFPC2 as part of the parallel observations of the Hubble Ultra Deep Field campaign, with BVi images from the Great Observatories Origins Deep Survey to identify a sample of Lyman break galaxies in the redshift range 2.0<=z<=3.5. We use recent stellar population synthesis models with a wide variety of ages, metallicities, redshifts, and dust content, and a detailed representation of the HI cosmic opacity as a function of redshift to model the colors of galaxies in our combination of WFPC2 and ACS filters. Using these models, we derive improved color selection criteria that provide a clean selection of relatively unobscured star-forming galaxies in this redshift range. Our WFPC2 F300W image is the deepest image ever obtained at that wavelength. The 10 limiting magnitude measured over 0.2arcsec^2^ is 27.5mag in the WFPC2 F300W image, about 0.5mag deeper than the F300W image in the Hubble Deep Field (HDF)-North.