A nonisotopic single-strand conformation polymorphism (SSCP) approach was employed to 'fingerprint'sequence variability in the expansion segment 5 (ES5) of domain IV and the D3 domain of nuclear ribosomal DNA within and/or among isolates and individual muscle (first-stage) larvae representing all currently recognized species/genotypes of Trichinella. In addition, phylogenetic analyses of the D3 sequence data set, employing three different tree-building algorithms, examined the relationships among all of them. These analyses showed strong support that the encapsulated species T spiralis and T nelsoni formed a group to the exclusion of the other encapsulated species T britovi and its related genotypes Trichinella T8 and T9 and T murreffi, and T nativa and Trichinella T6, and strong support that T nativa and Trichinella T6 grouped together. Also, these eight encapsulated members grouped to the exclusion of the nonencapsulated species T papuae and T zimbabwensis and the three representatives of T pseudospiralis investigated. The findings showed that nonencapsulated species constitute a complex group which is distinct from the encapsulated species and supported the current hypothesis that encapsulated Trichinella group external to the nonencapsulated forms, in accordance with independent biological and biochemical data sets.