  This contribution presents recent ZEUS results on proton structure functions
at HERA. The inclusive phi(1020)-meson cross section was measured, and it was
used to determine the s-quark content of the proton. The structure function F_2
was extracted using initial-state radiative events. Neutral and charged current
cross sections were used to extract the structure function xF_3 and measure the
mass of the W boson, respectively. A NLO QCD fit to ZEUS data and fixed target
cross sections was employed to determine the parton density functions of the
quarks and of the gluon inside the proton.
