The major purpose of this study is to analyze anterior and posterior temporalis muscle force recruitment and firing patterns in various anthropoid and strepsirrhine primates. There are two specific goals for this project. First, we test the hypothesis that in addition to transversely directed muscle force, the evolution of symphyseal fusion in primates may also be linked to vertically directed balancing-side muscle force during chewing (Hylander et al.  Am. J. Phys. Anthropol. 112:469–492). Second, we test the hypothesis of whether strepsirrhines retain the hypothesized primitive mammalian condition for the firing of the anterior temporalis, whereas anthropoids have the derived condition (Weijs  Biomechanics of Feeding in Vertebrates; Berlin: Springer-Verlag, p. 282–320). Electromyographic (EMG) activities of the left and right anterior and posterior temporalis muscles were recorded and analyzed in baboons, macaques, owl monkeys, thick-tailed galagos, and ring-tailed lemurs. In addition, as we used the working-side superficial masseter as a reference muscle, we also recorded and analyzed EMG activity of the left and right superficial masseter in these primates. The data for the anterior temporalis provided no support for the hypothesis that symphyseal fusion in primates is linked to vertically directed jaw muscle forces during mastication. Thus, symphyseal fusion in primates is most likely mainly linked to the timing and recruitment of transversely directed forces from the balancing-side deep masseter (Hylander et al.  Am. J. Phys. Anthropol. 112:469–492). In addition, our data demonstrate that the firing patterns for the working- and balancing-side anterior temporalis muscles are near identical in both strepsirrhines and anthropoids. Their working- and balancing-side anterior temporalis muscles fire asynchronously and reach peak activity during the power stroke. Similarly, their working- and balancing-side posterior temporalis muscles also fire asynchronously and reach peak activity during the power stroke. Compared to these strepsirrhines, however, the balancing-side posterior temporalis of anthropoids appears to have a relatively delayed firing pattern. Moreover, based on their smaller W/B ratios, anthropoids demonstrate a relative increase in muscle-force recruitment of the balancing-side posterior temporalis. This in turn suggests that anthropoids may emphasize the duration and magnitude of the power stroke during mastication. This hypothesis, however, requires additional testing. Furthermore, during the latter portion of the power stroke, the late activity of the balancing-side posterior temporalis of anthropoids apparently assists the balancing-side deep masseter in driving the working-side molars through the terminal portion of occlusion. Am J Phys Anthropol 128:35-56, 2005. © 2005 Wiley-Liss, Inc.