Several 15×15 cm 2 gaseous Micromegas chambers (MICROMEsh GAseous Structure) which consist of a conversion gap and an amplification gap separated by a thin grid have been extensively tested in low-intensity 10 GeV/c pion beam and high-intensity (up to 5×10 5 Hz/ mm 2) 100 GeV/c muon beam. The detector behaviour has been studied with respect to many parameters: conversion gaps of 1 and 3 mm, amplification gaps of 50 and 10 μm, an external magnetic field and many different filling gases. So far no effect of the magnetic field up to 1.3 T has been observed. The gas mixture argon + cyclohexane appears to be very suitable with gains above 10 5 and a full-efficiency plateau of 50 V at 340 V. With a conversion gap as small as 1 mm and an electronics with a threshold at 5000 electrons the efficiency reaches 96%. With the addition of CF 4 a time resolution of 5 ns (RMS) has been obtained. A spatial resolution better than 60 μ m has been observed with anode strips of 317 μ m pitch and was explained by transverse diffusion in the gas. Simulations show that with a pitch of 100 μ m and the appropriate gas a resolution of 10 μ m is within reach. This development leads to a new generation of cheap position-sensitive detectors which would permit high-precision tracking or vertexing close to the interaction region, in very high-rate environments.