ORGANELLE transport along microtubules is believed to be mediated by organelle-associated force-generating molecules1. Two classes of microtubule-based organelle motors have been identified: kinesin2-7 and cytoplasmic dynein8-12. To correlate the mechanochemical basis of force generation with the in vivo behaviour of organelles, it is important to quantify the force needed to propel an organelle along microtubules and to determine the force generated by a single motor molecule. Measurements of force generation are possible under selected conditions in vitro (for example, see refs 13 and 14), but are much more difficult using intact or reactivated cells. Here we combine a useful model system for the study of organelle transport, the giant amoeba Reticulomyxa 15, with a novel technique for the non-invasive manipulation of and force application to subcellular components, which is based on a gradient-force optical trap, also referred to as 'optical tweezers'16-19. We demonstrate the feasibility of using controlled manipulation of actively translocating organelles to measure direct force. We have determined the force driving a single organelle along microtubules, allowing us to estimate the force generated by a single motor to be 2.6 × 10-7 dynes. © 1990 Nature Publishing Group.