This study investigated the effects of explicit and implicit discourse marker (DM) instruction on English as a foreign language (EFL) learners' speech. Two different teaching methods were used over seven weeks of online English classes at a university in Japan. Participants were split into two groups, and in the first week, a pre-test comprising two speech tasks was administered to both groups. The groups were then given explicit or implicit feedback about the usage of target DMs using video clips and emails. The results of the post-test in Week 5 revealed that, although the frequency of the marker "for example" was significantly higher after explicit instruction, there was no significant difference in the frequency of the other target markers in both groups. However, some students' repertoires of DMs increased after explicit feedback. In Week 7, a speaking test including tasks similar to those in the pre- and post-tests was administered as a substitute for the delayed test. Although some easily accessible markers such as "and" and "because" were used in speech, both groups' repertoire was limited. Hence, explicit instruction on DMs may have a limited effect on EFL learners' speech.