Direct field evidence for links between volcanic rocks and contemporaneous subvolcanic intrusions are difficult to establish. Such links might, however, be inferred by making physical measurements on a youthful active volcano. Poás is a small composite volcano (elevation 2700 m, diameter 20 km) which has evolved to its present form by a sequence of caldera- and crater-forming episodes, possibly during the last 5 × 104 y. There has been a high level of historic activity from the active crater, which is 1 km in diameter and 300 m deep, and which contains a hot lake. Lavas and dominant pyroclastic rocks exposed in the active crater calc-alkaline basaltic andesites and andesites. Poás volcano is therefore typical of those in island arcs and young continental margins. Gravity measurements show that the volcano has a regional negative Bouguer anomaly, of amplitudes 100–200 gu, upon which is superimposed a closed positive anomaly, in the active crater area, about 2 km in diameter and with a maximum amplitude of 100 gu. This is thought to indicate a 1 km radius cylinder of solid rock of basaltic andesite or andesite composition with a shallow upper surface and extending to a depth of several kilometers. The concept of a large shallow magma chamber, with a diameter similar to that of the volcano, appears not to be appropriate for this volcano.