The attachment of muscle to skeletal elements in arthropods has been the sub ject of electron microscopical studies in insects (Auber, 1963 ; Shafiq, 1963 ; Lai Fook, 1967 ; Toselli and Pepe, 1968a, 1968b ; Caveney, 1969) , arachnids (Smith, JA¤lfors and Russell, 1969 ; Kuo, McCully and Haggis, 1971 ; Mazurkiewicz and Bertke, 1972) and crustaceans (Bouligand, 1962 ; Talbot, Clark and Lawrence, 1972 ; Koulish, 1973) . The results of these studies reveal that the attachment region is similar in structure throughout the arthropods that have been examined. The attachment may be made directly to the exoskeleton, but in most cases is achieved through a highly specialized epidermal cell, usually called a tendinal cell, interposed between a muscle fiber and the cuticle. The most conspicuous feature of the tendinal cells is their tremendous concentration of oriented microtubules which extend the length of the cell and join desmosomal and hemidesmosomal junctions formed between muscle and tendinal cell and tendinal cell and apodeme respectively. At the present time, no studies of muscle attachments utilizing electron micro scopical techniques have been reported for a fourth major class of arthropods, the Merostomata. Such a study of members of this group which includes the oldest extant arthropods, the horseshoe crabs, would be particularly interesting in view of their long evolutionary history. This report is concerned with the ultrastruc tural details of the attachment of propodite flexor muscle fibers to the exoskeleton in Liinulus polyphenius (L.). The results show that for the most part, these muscle attachments in horseshoe crabs follow the pattern described for other mem bers of the Arthropoda.