This paper explores textual meanings realised in the theme resources of a Vietnamese lower secondary school science textbook – Sinh h?c 8 (Biology 8). The theoretical framework employed for analysis is Systemic Functional Linguistics. The data for analysis is ten texts selected randomly from the textbook. The units of analysis are major independent clause simplexes and hypotactic clause complexes which have a subordinate clause in initial position. The research has brought to light a number of significant findings of which eight stand out. First, all five types of general Themes: simple Theme, multiple Theme, clausal Theme, unmarked Theme, and marked Theme are employed in the texts. Second, of the four mood function Themes: Subject Theme, Predicator Theme, Complement Theme, and Adjunct Theme, Subject Theme takes up the largest proportion. Third, of the three mood function simple Themes: simple Subject Theme, simple Predicator Theme and simple Adjunct Theme, simple Subject Theme has the highest number. Fourth, between textual + topical Theme and interpersonal + topical Theme, textual + topical Theme predominates. Fifth, of the three mood type simple Subject Theme: simple Subject Theme in declarative clause, simple Subject Theme in imperative clause and simple Subject Theme in interrogative clause, simple Subject Theme in declarative clause accounts for the highest number. Sixth, between simple Predicator Theme and multiple Predicator Theme, simple Predicator Theme has a much higher proportion. Seventh, of the nine types of simple Adjunct Theme examined, six are found in the texts, among which spatial location Adjunct Theme takes the highest number. And Eighth, Complement Theme, multiple Adjunct Theme, textual + interpersonal + topical Theme, interpersonal + topical/Subject Theme, matter Adjunct Theme, role Adjunct Theme, and stance Adjunct Theme are not found in the texts. It is concluded that Vietnamese textbook writers have employed quite a variety of theme resources to construct textual meanings and to create coherence and continuity of their texts.