Mastery of idioms in a second language is an important part of learners' lexical and cultural competence. This study provided insights into the processes that partake in interlanguage idiomatics. It also looked at the role of participants' first language in the comprehension and production of idioms in their second language. Participants were 31 Saudi graduate students' who were learners of English. They were tested in receptive and productive knowledge of 15 English idioms of three types of idiomatic expressions: a) English idioms which have their exact equivalents in Arabic; b) English idioms which are similarly expressed in both languages, and c) English idioms which have no counterparts in Arabic. The data were examined through statistical analysis. Results showed that identical idioms were the easiest to comprehend and produce, followed by similar idioms. Different idioms were the most difficult to comprehend and produce and showed the highest interference from the first language. In addition, a rather unexpected negative correlation was found between participants' length of stay in the US and their knowledge of English idioms. These findings offer insightful implications for the teaching and learning of second language idioms. Most importantly language teachers and researchers should be aware that the acquisition of idioms is influenced by the similarities and differences between idioms in learners' L1s and the target L2. This suggests that building learners' idiomatic knowledge in a second language should be done in a systematic and persistent way. In more general terms, the study's results confirm the trends and patterns reported in previous research about the acquisition of second language idiomatic competence and its important role for the effective comprehension and production of the target language.