_Experimental hosts._--_Blatta orientalis_, _Blattella germanica_, _Parcoblatta pensylvanica_, _Parcoblatta virginica_, _Periplaneta americana_, and _Supella supellectilium_, U.S.A. (Oswald, 1958): The larvae underwent normal development in all species of cockroaches except _B. orientalis_ and _P. americana_ in which cysts developed that contained a reddish-brown pigment; larvae in such cysts were dead or dying. Eggs of _R. coloradensis_ hatched in the midgut of _B. germanica_ and first-stage larvae entered the hindgut epithelium within 24 hours. The larvae underwent two molts within a cyst formed by tissues of the host's gut, the second molt occurring during the twelfth or thirteenth day. In _Parcoblatta_, cysts were found free in the body cavity as well as attached to the hindgut. In _B. germanica_ and _S. supellectilium_ the cysts remained attached to the hindgut. Usually over 20 cysts developed in each infected _Parcoblatta_; fewer than 10 per insect developed in the other species. Larvae became infective to the definitive host, the white-footed mouse [_Peromyscus leucopus noveboracensis_ (Fischer)], as early as the tenth day. Family SPIRURIDAE * =Agamospirura parahormeticae= Pessôa and Corrêa, 1929 _Natural host._--_Parahormetica bilobata_, Brazil (Pessôa and Corrêa, 1929). * =Gongylonema ingluvicola= Ransom, 1904 _Experimental host._--_Blattella germanica_, U.S.A. (Cram, 1935). * =Gongylonema neoplasticum= (Fibiger and Ditlevsen, 1914) Ransom and Hall, 1916 _Natural hosts._--_Blatta orientalis_, Netherlands (Baylis, 1925). _Blattella germanica_, U.S.A. (Hitchcock and Bell, 1952). _Periplaneta americana_, Denmark and St. Croix (Fibiger, 1913, 1913a; Fibiger and Ditlevsen, 1914); Netherlands (Baylis, 1925); Argentina (Bacigalupo, 1930); England (Leiper, 1926); South Africa (Porter, 1930); U.S.A. (Hitchcock and Bell, 1952); Formosa (Yokagawa, 1924, 1925, 1925a). _Periplaneta australasiae_, Formosa (Yokagawa, 1924, 1925, 1925a). _Experimental hosts._--_Blattella germanica_, Denmark (Fibiger and Ditlevsen, 1914); U.S.A. (Hitchcock and Bell, 1952); France (Brumpt, 1949). _Blatta orientalis_, Denmark (Fibiger and Ditlevsen, 1914). _Periplaneta americana_, Denmark and St. Croix (Fibiger, 1913; Fibiger and Ditlevsen, 1914); U.S.A. (Hitchcock and Bell, 1952). * =Gongylonema pulchrum= Molin, 1857 _Experimental hosts._--_Blattella germanica_, U.S.A. (Ransom and Hall, 1915, 1916, 1917; Stiles and Baker, 1927; Schwartz and Lucker, 1931; Lucker, 1932; Alicata, 1934a, 1935); Europe (Baylis et al., 1925, 1926, 1926a; Sambon, 1926). _Parcoblatta_ sp., Alicata (1934, 1935). * =Gongylonema= sp. _Natural host._--_Periplaneta americana_, Brazil (Magalhães, 1900); Algeria (Seurat, 1916); England? (Leiper, 1926). * =Microtetrameres helix= Cram, 1927 _Experimental host._--_Blattella germanica_, U.S.A. (Cram, 1934). * =Protospirura bonnei= Ortlepp, 1924 _Natural host._--_Leucophaea maderae_, Venezuela (Brumpt, 1931).