The juvenile stage of the marine nematomorph Nectonema agile Verrill infects the natantian decapod crustacean Palaemonetes vulgaris in the vicinity of Woods Hole, Massachusetts. It is suggested that P. vulgaris serves as a specific host in this locality, whereas different decapods, mainly anomurans, may be normal hosts for this protelean parasite in Europe. Information concerning the ecology of the genus Nectonema, the only marine representative of the class Nematomorpha, is scanty. This report establishes the natantian decapod crustacean Palaemonetes (Palaemonetes) vulgaris Say, 1818 as a host for juvenile stages of Nectonema agile Verrill, 1879, and includes observations on host-parasite relations. Verrill (1879) collected free-living adult Nectonema agile from Vineyard Sound, Massachusetts. Ward (1892) described the anatomy, and inferred that there must be a host for juvenile stages. A year later, after observing a single worm in a preserved shrimp, Ward (1893) stated that Palaemonetes (species not given) was the juvenile host, but called for confirmation of this observation. Adult horsehair are taken fairly often at night lights in the Woods Hole area, but the life cycle of this organism has not previously been known. Perez (1927) definitely established that juvenile stages of Nectonema are parasitic in crustaceans. He reported Nectonema infections in 12 of 308 Anapagurus hyndmanni in Roscoff Bay, France. Brinkman (1930) reported single instances of Nectonema infections in Pontophilus norvegicus and Eiupagurus pubescens after examination of several thousand individuals. Both Perez and Brinkman concluded that Ward's report (1893) may be an example of Nectonema developing in an occasional host. H. and L. Nouvel (1935) reached the same conclusion after studying two N. agile individuals collected during an extenReceived for publication 10 March 1967. * Present address: Department of Zoology, University of California, Berkeley, California 94720. t Contribution No. 1914 from this Institution. sive survey of the marine littoral palaemonid, Le nder serratus, at Roscoff. Perez (1935) found roughly equal infections of N. agile (c . 5%) in an anomuran, Eupagurus bernhardus, and in an oxyrhyncoid brachyuran, Macropodia rostrata, after extensive sampling in Morlaix Bay, France. Feyel (1936) distinguished four species of Nectonema: N. agile; N. munidae Brinkman; N. melanocephalum Nierstrasz; and N. svensksundi Sixten Bock. Decapod hosts for the last two species have not yet been reported. My observations show that Palaemonetes vulgaris is a normal host for N. agile, and also suggest that maturing juvenile worms may not infect other decapod species in the same waters. A single worm was first noticed in an adult male P. vulgaris collected from Great Pond, near Falmouth, Massachusetts, on 18 June 1963. On 21 June 1964, another collection there yielded three infected hosts from a sample of about 100 shrimp. In more than 30 samples (over 5,000 individuals) of this species from Quissett Harbor, an inlet on Buzzard's Bay, from 19 June to 15 September 1963, 23 parasitized male and 14 parasitized female shrimp were noted. In one instance two worms were found within one host. However, about half of the shrimp observed were juveniles (spawned that summer), and none of these appeared infected. During the same time period no worms were noted in 18 similar samples (about 2,000 individuals) of Crangon septemspinosa from the same locality. Occasional collections of the smaller caridean Hippolyte zostericola did not yield any Nectonema. A small sample (ca. 100) of the hermit crab Pagurus longicarpus was examined with negative results. It appears that in the Woods