The organization of the human mental lexicon has been explored for many years through various psychological experiments. This paper focuses on exploring and comparing the first- and the second-language mental lexicon with the help of a word association test. Native and non-native (Croatian) speakers of English provided their associations to twenty words written in English, which were then analysed according to a modified version of a categorization system proposed by Tess Fitzpatrick. This categorization system allowed for a thorough comparison of response type tendencies in L1 and L2. Furthermore, a separate analysis of the responses produced by lower- and higher-English-proficiency non-native speakers was used to explore the effect of language proficiency on the results. Lastly, the effects of concreteness and imageability of cue words on associations were investigated by comparing the responses elicited by words of different levels of concreteness and words of different levels of imageability. The paper attempts to provide an insight into the organization of an L1 and an L2 mental lexicon and how their similarities and differences are affected by L2 proficiency. Additionally, it includes indications of the influence of concreteness and imageability levels of cue words on the association links activated both in L1 and L2.