An equation for the specific storage of an inland artesian aquifer of infinite extent is developed from a consideration of the tidal dilatation of an idealized well-aquifer system. It was found that for a particular tidal frequency the specific storage Ss is related to tidal water level and barometric pressure fluctuations according to the relationship Ss=-0.5 W2 cos &PHgr;e/{agwcos&PHgr;w+Bdhb cos &PHgr;b>}, where W2, dhw, dhb are the amplitudes of the periodic tide-generating potential, the water level fluctuations in the well, and the barometric pressure fluctuation; &PHgr;e, &PHgr;w, and &PHgr;b are the phase angles between the tide-generating potential and the earth tide dilatation, the water level fluctuation in the well, and the barometric pressure fluctuations; B is the barometric efficiency; a is the radius of the earth; and g is the acceleration of gravity. The earth tide dilatation phase angle &PHgr;e?0.0 can be assumed. Two arbitrary numerical methods are devised for estimating barometric efficiency from aperiodic water level and barometric pressure fluctuations. If the specific storage and barometric efficiency are known, in situ values of porosity n and matrix bulk modulus Em can be calculated by the following expressions: n={1¿<1-(4 Ss Ew2>/&rgr;g)>1/2}/2 (1-B) and Em=&rgr;g{1¿<1-(4 Ss Ew2>/&rgr;g)>1/2}/2 Ss (1-B) where Ew is the bulk modulus of water and &rgr; is the density of water. Tidal water level fluctuations were recorded in three observation wells in Montgomery County, Virginia. Physically reasonable values of specific storage, barometric efficiency, porosity, and matrix bulk modulus were calculated, but the accuracy of these values is limited by the imprecise measurements of water level and barometric pressure and the possibility of leakage from imperfectly confined aquifers.