-In northern Wisconsin lakes, the crayfish Orconectes rusticus is replacing 0. propinquus, a previous invader, and 0. virilis, a native crayfish. Because chela size dictates success in aggressive encounters, predatory defense and reproduction, interspecific size differences could lend insight to this ongoing replacement. To quantify chela size differences among these three species, we measured chela lengths and widths of Form II (FII, nonreproductive) males, Form I (Fl, reproductive) males and females. For Fl and FII males, both 0. rusticus and 0. propinquus had larger chelae than similar-sized 0. virilis. Females differed only slightly in chela size. Females had smaller chelae than their male conspecifics. Chela size differences, coupled with differential body size, likely contributes, via differential predatory susceptibility and reproductive success, to the replacement of 0. propinquus and 0. virilis by 0. rusticus. INTRODUCTION The crayfishes, Orconectes rusticus, 0. propinquus and 0. virilis occur within littoral zones of N temperate lakes, often in sympatry (Crocker and Barr, 1968; Berrill, 1978; Capelli and Capelli, 1980; Corey, 1988). In northern Wisconsin lakes, 0. virilis is endemic; both 0. propinquus and 0. rusticus are exotics (Capelli, 1982). First to invade these lakes, 0. propinquus appears to displace 0. virilis, whereas 0. rusticus, the most recent invader, displaces both 0. virilis and 0. propinquus (Capelli, 1982; Lodge et al., 1986). Mechanisms explaining this replacement remain elusive, though predation, competition and reproductive interference all have been implicated (Capelli and Capelli, 1980; Capelli, 1982; Capelli and Munjal, 1982; Lodge et al., 1986). Herein, we quantify interspecific morphological differences that potentially could contribute to this replacement. Orconectes rusticus, 0. propinquus, and 0. virilis share similar gross external morphological characteristics (Capelli and Capelli, 1980; Hobb and Jass, 1988). However, chela size differs among these crayfishes. Chelae comprise a substantial portion of crayfish dry weight (Stein, 1976) and though energetically costly to grow, large chelae benefit crayfish in a variety of ways. Crayfish with large chelae are less susceptible to fish predation (Stein, 1976) and able to dominate similar-sized crayfish having small chelae (Bovbjerg, 1956; Stein, 1976; Snedden, 1990; Garvey, 1992). Furthermore, males with large chelae are better able to secure and hold females during copulation than similar-sized males with small chelae (Stein, 1976; Snedden, 1990). Though chela size is important, 0. rusticus and 0. virilis grow larger than 0. propinquus (Corey, 1988; Olsen et al., 1991). Because large crayfish are less susceptible to predation and able to dominate small crayfish (Stein and Magnuson, 1976; Stein, 1977), chela plus body size differences likely combine to influence interspecific interactions among these crayfishes. By quantifying chela length and width for these crayfishes, we can assess chela size differences. We then can evaluate the effect of chela size coupled with body size differences on the replacement of 0. virilis and 0. propinquus by 0. rusticus.