The paper describes a series of in-isolation cyclic load-extension tests performed on HDPE,and PET geogrid specimens. The specimens were tested at five different loading frequencies: 0.1, 0.5, 1.0, 2.0 and 3.5 Hz, and over a range of load amplitudes. The load-strain behavior of the geogrid materials is characterized using two distinct features of the cyclic response: 1) non-linear hysteresis unload-reload loops; and 2) a load-strain cap that contains all hysteresis curves. The cyclic load-strain response of the HDPE and PET geogrid specimens is demonstrated to be significantly different from the static response of the same materials tested according to the current ASTM D 4595 method of test. The load-strain caps for HDPE geogrid specimens are found to be very sensitive to the loading frequency and loading history while those of the PET geogrid appear to be relatively insensitive. The hysteresis load-strain behavior of both types of geogrids depends strongly on the load level and loading frequency.