Information about site-specific crop parameters such plant height, coverage, and biomass density is important for optimising crop management and harvesting processes. Sensors for measuring crop parameters with an acceptable accuracy and high reliability at a reasonable price are essential prerequisite to obtain this information. In the last years laser rangefinder sensors are adopted in many industrial applications. In agricultural engineering the potential of laser rangefinders for measuring crop parameters is nearby unexploited. This paper reports the design and the performance of a triangulation laser based system for measuring crop biomass density under field conditions in the vegetation period 2005. The main parameter for potential assessment was the coefficient of determination R^2 of the relation between crop biomass density (related on fresh and dry matter) and the mean height of the laser reflection point in crop stands from oilseed rape, winter rye, winter wheat, and grassland. In oilseed rape, winter rye and winter wheat R^2 was calculated in the range from 0.93 to 0.99. Caused by variable morphologic features on grassland R^2 was reduced in the range from 0.88 to 0.94. Taking into account that large crop stand areas can be sensed by a laser rangefinder in a short time, a high potential for site specific crop management can be concluded.