We present the analysis of FUSE observations of the metal-deficient dwarf galaxy I Zw 18 . We measured column densities of H i , N i , O i , Ar i , Si ii , and Fe ii . The O i /H i ratio ( \log ( O i /H i ) = -4.7 ^ { +0.8 } _ { -0.6 } ) is consistent with the O/H ratio observed in the H ii regions ( all uncertainties are 2- \sigma ) . If the oxygen is depleted in the H i region compared to the H ii regions , the depletion is at most 0.5 dex . This is also consistent with the \log ( O/H ) ratios \sim - 5 measured with FUSE in the H i regions of other blue compact dwarf galaxies . With \log ( N i /O i ) = -2.4 ^ { +0.6 } _ { -0.8 } , the measured N i /O i ratio is lower than expected for primary nitrogen . The determination of the N ii column density is needed to discriminate between a large ionization of N i or a possible nitrogen deficiency . The neutral argon is also apparently underabundant , indicating that ionization into Ar ii is likely important . The column densities of the other \alpha -chain elements Si ii and Ar i favor the lower edge of the permitted range of O i column density , \log ( N _ { cm ^ { -2 } } ( O i ) ) \sim 16.3 .