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The CLS:Argos location and data collection system is used widely by researchers tracking the movements of animals. The accuracy of the Argos location classes is undefined for most Argos locations for studies involving tracking animals. Published empirical data on the accuracy of animal-mounted transmitters are limited to stationary units. The accuracy of the positions is defined by Argos, except for location classes (LC) = 0, A, B, and Z. The distinction between ‘accuracy’ and ‘precision’ is discussed using field measurements from 24,466 Argos records collected throughout the world, but mostly in the Southern Hemisphere, between 1992 and 2001. Factors affecting the defined ‘accuracy’ and ‘precision’ are identified from this analysis. Neither the transmitter’s age, nor its attachment to a bird degraded its performance. However, the performance of transmitters in terms of the locations they provided was affected when the objects they were attached to moved rapidly, and, with 1 platform transmitter terminal (PTT), by altering of the proportion of location classes within the experiment, but not the ‘precision’ of the classes (LC = 3, 2, 1, and A). The ‘precision’ (rounded, measured as 1 SD of the mean of the distance of the location from the actual position occupied by the transmitter, for ”Location Classes” 3, 2, and 1 was