This study aims to analyse if the modal auxiliary verb must and the semi-modal auxiliary verb have to differ in frequency and meaning, deontic and epistemic, in the registers newspapers and spoken conversation in the BNC Baby. Furthermore, this paper analyses whether the verb collocates of must and have to could affect the meaning, deontic and epistemic, in the two registers. The taxonomy adopted for deontic and epistemic meanings of must and have to is based on past research. The study has taken into consideration the taxonomies of the meaning of modal auxiliaries by Quirk et al. (1985), Biber et al. (1999), and Huddleston and Pullum (2002). The frequency of must and have to was found to be very similar in the register newspapers in the BNC Baby, must amounting to 48 percent and have to 52 percent, while the frequency of have to was much greater in the register spoken conversation in the BNC Baby, with have to reaching 71 percent and must only 29 percent. Must was found to convey a deontic meaning more often in the register newspapers, where the distribution was 70 percent deontic and 30 percent epistemic, while must was found to convey an epistemic meaning more often in the register spoken conversation, where the distribution was 85 percent epistemic and 15 percent deontic. Conversely, have to conveyed mostly a deontic meaning in both registers, with a distribution of 100 percent deontic in newspapers and 98 percent deontic in spoken conversation. Finally, it was found that the verb collocates of must and have to could affect the deontic and epistemic meaning of must and have to in the two registers studied.