Multilingual learners (MLs) bring numerous resources to science learning contexts. In classrooms, multilingual learners have expansive linguistic repertoires to draw upon for information-seeking (Gynne & Bagga-Gupta, 2015), collaboration (French, 2016; Garcia & Sylvan, 2011), and learning (Vygotsky, 1978). However, within US secondary science classrooms, traditionally English-centric spaces, MLs' resources may go unrecognized for a number of reasons, including monolingual ideologies (Wiley & Lukes, 1996), assimilationist discourses (de Jong, 2013), and language-as-problem orientations (Ruiz, 1984). Multimodal composing provides opportunities to engage with and learn through modes in addition to language (e.g., image, sound, realia). While composing through multiple modes in classrooms, MLs have access to a wide range of nonlinguistic resources for the learning of content (Goulah, 2017; Suarez, 2020; Zheng et al., 2014), language (Goulah, 2017; Gynne & Bagga-Gupta, 2015; Pacheco & Smith, 2015), and composing (Ajayi, 2015; D'warte, 2014; Dalton et al., 2015). Multimodal composing has been shown to hold benefits for MLs in other content areas (Baize, 2019; Kane, 2018; Smith, 2019; Smith et al., 2019; Staurseth & Haland, 2018) but few studies have explored its affordances within secondary biology (notable exceptions Castek & Dwyer, 2018; Pierson et al., 2021). This dissertation explored multilingual learners' use of various modes and participation in science practices within a secondary marine biology classroom through a comparative case study. Findings of this dissertation study demonstrate how multilingual learners used various modes while multimodal composing to: (1) interact with classmates, (2) position themselves, and (3) enact their vision of engaging in science. Further, findings also show how multilingual learners participated in science and engineering practices: (1) by leaning into their strengths, (2) in accordance with how they positioned themselves, (3) and in accordance with their understandings of science. Findings from this work have implications for current conversations in multimodal composing research, science education for multilingual learners, and language education. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]