Confinement of light to small volumes has important implications for optical emission properties : it changes the probability of spontaneous emission from atoms, allowing both enhancement and inhibition. In photonic-bandgap (PBG) materials(1-4) (also known as photonic crystals), light can be confined within a volume of the order of (lambda/2n)(3), where lambda is the emission wavelength and n the refractive index of the material, by scattering from a periodic array of scattering centres. Until recently(5,6), the properties of two-and three-dimensional PBG structures have been measured only at microwave frequencies. Because the optical bandgap scales with the period of the scattering centres, feature sizes of around 100 nm are needed for manipulation of light at the infrared wavelength (1.54 mu m) used for optical communications. Fabricating features this small requires the use of electron-beam or X-ray lithography. Here we report measurements of microcavity resonances in PBG structures integrated directly into a submicrometre-scale silicon waveguide. The microcavity has a resonance at a wavelength of 1.56 mu m, a quality factor of 265 and a modal volume of 0.055 mu m(3). This level of integration might lead-to new photonic chip architectures and devices, such as zero-threshold microlasers, filters and signal routers.