This paper examines the grammatical errors in the written productions of senior secondary school students in/from Lycée Amadou Kouran Daga (henceforth LAKD), Zinder, Niger. It specifically seeks to describe, identify and classify the grammatical errors with a view to finding out the reasons of their occurrence in the students’ writings. The participants of the present study consist of 30 students (14 male and 16 female students) learning English as a foreign language. The data for the investigation were collected by means of an essay writing which required the participants to write a three-paragraph essay on one of the four narrative and descriptive topics provided. The collected data were then analyzed using Error Analysis and the simple descriptive statistics consisting of frequencies and percentages. The analysis revealed a total of 410 grammatical errors in the participants’ writing samples ranked by order of frequency and categories as follows: errors on tenses, spelling errors, agreement errors, preposition and article errors, verb errors, use of nouns, and adjectives. The analysis also showed that male participants committed more grammatical errors than their female counterparts. Further, using Surface Strategy Taxonomy, the findings revealed that the participants committed all the four types of grammatical error. Misformation errors were the most dominant type, followed by omission errors, addition errors, and misodering errors.