The role of explicit and implicit acquisition of grammatical rules in second language learning was examined by assessing high school students performance after several years of study on the correct use of the Spanish verbs "ser" and "estar". These two verbs are essentially equivalent in use to the English verb "to be," but there is a complex set of statements that specifies when each is to be used. A 100-item two-choice test in which students from several different high schools receiving instruction in the explicit principles of these two Spanish verbs or receiving immersion training in which no instruction in their grammatical principles was given were examined for the following five factors: (a) general achievement level, (b) comparison of grade level achievement, (c) time allotted to complete the 100-item test, (d) grammatical instruction prior to taking the 100-item test, and (e) comparison of instructional methodology. The results indicated that students receiving explicit instruction in the rules of "ser" and "estar" beyond the first year of study achieved slightly higher scores, but that the level of achievement was considered to be poor. Differences between groups for the remaining factors were not significant. It was concluded that successful application of explicitly learned grammatical rules, such as the rules for "ser" and "estar", may not be achievable in the time frame of a two-year high school foreign language course. Considerations regarding implicit and explicit acquisition of grammatical rules in second language learning were presented.