The p T -differential inclusive production cross section of the prompt charm-strange meson D s + in the rapidity range | y | < 0.5 was measured in protonaproton collisions at s = 7 TeV at the LHC using the ALICE detector. The analysis was performed on a data sample of 2.98 X 10 8 events collected with a minimum-bias trigger. The corresponding integrated luminosity is L int = 4.8 nb - 1 . Reconstructing the decay D s + a I pi + , with I a K - K + , and its charge conjugate, about 480 D s +/- mesons were counted, after selection cuts, in the transverse momentum range 2 < p T < 12 GeV / c . The results are compared with predictions from models based on perturbative QCD. The ratios of the cross sections of four D meson species (namely D 0 , D + , D a + and D s +) were determined both as a function of p T and integrated over p T after extrapolating to full p T range, together with the strangeness suppression factor in charm fragmentation. The obtained values are found to be compatible within uncertainties with those measured by other experiments in e + e - , ep and pp interactions at various centre-of-mass energies.