A study is presented that examines the null-subject parameter (NSP) and that seeks to attain the following objectives: (1) to assess the validity of the implicational hierarchy for the NSP, especially as proposed by Liceras (1989); and (2) to determine if there is any evidence to support the theory of the Weaker Logical Problem of Acquisition (WLPA). The study involved 24 students, learning Spanish as a foreign language, who were enrolled in a third semester university course. Information regarding the properties of NSP and a discussion of previous second language studies are presented. An analysis of the results is provided of a 43-item grammatical-judgment test designed to assess the status of the three properties of the NSP in the learners' interlanguage, and of a collaborative judgment task. 19 references. (GLR)