  Inclusive phi-meson production in neutral current deep inelastic e+p
scattering has been measured with the ZEUS detector at HERA using an integrated
luminosity of 45 pb^{-1}. The phi mesons were studied in the range 10<Q2<100
GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted
kinematic regions in the transverse momentum, p_T, pseudorapidity, eta, and the
scaled momentum in the Breit frame, x_p. Monte Carlo models with the
strangeness-suppression factor as determined by analyses of e+e- annihilation
events overestimate the cross sections. A smaller value of the
strangeness-suppression factor reduces the predicted cross sections, but fails
to reproduce the shapes of the measured differential cross sections.
High-momentum phi mesons in the current region of the Breit frame give the
first direct evidence for the strange sea in the proton at low x.
