This paper describes some observations of flow boiling of water at near-atmospheric pressure in a single vertical channel of rectangular cross-section 2 mm x 1 mm, heated on three sides over a length of 248 mm, with a window on the fourth side. Measurements have been made of the fluctuating wall temperatures and pressures at several stations along the channel, with simultaneous high-speed video recordings of the flow patterns at a mass flux of 134 kg m(-2) s(-1) and heat fluxes of 50-100 kW m(-2). The frequency spectra of the fluctuations depend on the compressibility of the inlet flow to the channel. It is shown that the processes of initiation of boiling and of heat transfer differ from flow boiling in large channels. The acceleration of liquid slugs by confined bubbles causes pressure pulses to propagate the full length of the channel. Heat transfer is distributed in time between convection and nucleate boiling. The experimental limitations are discussed.