Copulation of Hyalomma excavatum and Rhipicephalus sanguineus belonging to the Metastriata group of the Ixodidae has been studied. Methods of observation and inducing copulation off the host are described. Similarities and differences in male behavior of the Ixodidae have been compared with Argasidae. It has been found that the pattern of copulation in Metastriata differs not only from that of the Argasidae, but also from the Prostriata which belong to the same family (i.e., the Ixodidae). Furthermore, it has been observed that the fourth article of the male palp is essential for copulation, since males without it do not copulate. Copulation in ticks has been studied mainly in Argasidae (Nuttall and Merriman, 1911; Robinson, 1942; Feldman-Muhsam, 1969); in Ixodidae few observations are available (Bodkin, 1918; Moorhouse, 1966). This is probably due to the fact that Argasidae copulate eagerly in the laboratory, while in Ixodidae copulation is very difficult to obtain. Arthur (1962) claims that "coupling and insemination in these groups (i.e., genera belonging to the Ixodidae, Metastriata group) require very much more investigation, as Nuttall and Merriman indicated as far back as 1911." Our observations show that there are both similarities and differences in the pattern of copulation between the two families. In the following, our present observations on Ixodidae will be compared with our previous observations (Feldman-Muhsam, 1969; Feldman-Muhsam et al., 1970) on copulation in Argasidae. Ixodid ticks of the Metastriata (or Amblyomminae) group are known to copulate on their hosts. Adults are frequently found attached to the host in couples, a male and a female feeding venter to venter, and in this position they copulate in the middle of their meal. The female completes engorgement only after copulation. If a female does not copulate, she does not fully engorge and may remain on the host awaiting a male for several weeks, instead of several days if she has been fertilized. The ixodid female which oviposits only once in her lifetime theoretically needs only one supply of sperm. Counts of two spermatophores of Hyalomma excavatum amounted to 80,000 and Received for publication 8 December 1970. * Supported by NIH grant AI-07711; U. S. Public Health Service. 120,000 sperms, respectively, whereas the number of eggs oviposited by one female generally does not exceed 10,000. Ticks of the Prostriata (Ixodidae) group may copulate both on and off the host (Balashov, 1967). Argasid ticks copulate always off the host. The general pattern of reproduction-subject to variations from one species to another-requires the female to copulate and to feed anew before each oviposition. The female oviposits several times during her life. Therefore the argasid male copulates many times: fed Ornithodoros tholozani have been observed in our laboratory to copulate once a day and 0. savignyi twice a day. Even starved males of O. tholozani and 0. savignyi copulated up to 21 and 32 times, respectively, during their life (Feldman-Muhsam, 1967a). The difference between Ixodidae and Argasidae in location and number of copulations, under natural conditions, seems to be the reason why, in the laboratory, it is easy to induce copulation in Argasidae and difficult in Ixodidae.