This study investigates slang and mobilephone message language use by senior secondary school students in Kwara State, Nigeria. Ninety students (fifteen males and fifteen females each) were selected each of the three Senatorial Districts of Kwara North, Kwara Central and Kwara South through a stratified random sampling technique. The instrument used was a thirty-item researcher-designed standard British English, slang and mobilephone message language use questionnaire which was given to two English language experts for its validity. The instrument was subjected to test re-test technique of three weeks interval after the first administration of the instrument. The scores obtained in the two administrations were correlated using Pearson Product Moment Correlation statistic 0.05 significant level, which resulted in 0.61reliability index. The findings revealed that high significant differences existed between standard British English and slang use by the students, as well as high significant differences between standard British English and mobilephone message language use by the students. As a result of the findings and the conclusion, it was recommended that teachers should as much as possible discourage the use of slang and mobilephone message language use among students by constantly engaging the students in oral drill and short-gun tests to awaken their standard British grammatical consciousness. It was also recommended English experts should be invited to give talks on the danger inherent in the use of slang or mobilephone message language, among other recommendations.