  A search for very-high energy cosmic muon neutrinos from unresolved sources
is presented using data collected by the ANTARES neutrino telescope. Data
corresponding to 334 days of equivalent live time show that the observed number
of events is compatible with the expected number of background events. A 90%
c.l. upper limit on the diffuse $\nu_\mu$ flux is set at $E^2\Phi_{90%} = 5.3
\times 10^{-8} \ \mathrm{GeV\ cm^{-2}\ s^{-1}\ sr^{-1}} $ in the energy range
20 TeV -- 2.5 PeV.
