This paper reports the findings of a study of the acquisition of relative clauses in Chinese, Japanese, Persian, German, and Portuguese by first and second year students of these languages with English as their first language. Various structures were tested with the aim of separating the features of interlanguage which may be attributed to first language interference from those which may be explained as being universal to second language acquisition. Among these structures were use of resumptive pronouns in the relative clause, deletion of relativizers, stranding of prepositions, deletion of prepositions, and piedpiping structures. In line with the work of Gass and Ard (1980) and Hyltenstam (1981), the present paper examines the explanatory value of the accessibility hierarchy of Keenan and Comrie (1977) as applied to the ease or difficulty of the acquisition of relativization of various syntactic types, and expands this study by including data from the acquisition of relative clause types not examined in previous works. Also tested were structures involving different strategies for the parsing and recognition of relative clauses, in order to determine which are employed most frequently and most successfully by second language learners, and structures with correct and incorrect deletion of prepositions, in order to see whether the loss of information involved would be a constraining factor.