A small vertical seismometer whose inertial mass is supported by a leaf spring has been developed as a replacement for conventional long-period (LP) seismometers. The mechanical sensor has a virtually infinite natural period and is operated in a force-balance feedback configuration with an overall response identical to that of a 20-sec LP seismometer. Main considerations in the design were economic production and efficient shielding against environmental disturbances. The sensor is thermally coupled to the ground and protected from atmospheric pressure variations by a vacuum bell. This allows increasing the useful gain at very long periods by two orders of magnitude compared to a standard LP seismograph. The instruments resolve ground noise at least from a 0.3- to 300-sec period (typically, from about 0.1 to 3000 sec) and have a dynamic range of 140 dB. Leaf-spring seismometers have been tested since 1976 as part of wideband, LP, and very LP seismographs. Matching horizontal sensors are also available. This paper discusses the design principles and the initial calibration, and presents test results and typical seismograms.