Previous research done in sociophonetic variation of second language speakers has often looked at constraints of formality affecting degree of foreign accent and how this degree of formality can have effects on what speaking styles speakers choose to employ. Furthermore, other social constraints of convergence and divergence of speech affect speaker speaking style. However, no known previous research has examined interdental fricative /θ ð/ substitution based on each speaker's interlocutor. This study explores second language speakers' English interdental fricative substitution sounds in terms of sociophonetic variation of formality and speaker interlocutor(s). Five native language pairs of Arabic, Cantonese, French, Portuguese, and Vietnamese origin were part of the study, comprising ten participants in total. The study finds age of English onset, as verified by the literature, to be the most determining factor for accurate articulation of these marked fricatives. However, other constraints for substitution choice are at hand including phonological limitations and estimated linguistic experience based on demographic information given by survey participants. The primary aim of the study is to associate some of the interdental fricative substitutions with a social variable. Data for the study include recordings of each participant reading a poem by him/herself, a dialogue with the other same native language participant, and a dialogue with a native speaker of American English. The data analysis examined the replacement sounds in terms of native language background, linguistic experience variables, and phonological constraints. In addition, quantities and ratios of specific replacement sounds for each participant per recording and per native language pair were compared and contrasted to find if speech accommodation theory (SAT), as proposed by Giles et al. (1991), played a role in any of the participants' choices for substitution. The study finds both convergence and divergence of interdental fricative substitutions to be characteristic of speakers with less linguistic experience in English. An additional stronger finding is that most participants' most common sound substitutions for the voiced and voiceless interdental fricatives were independent in place and manner, the voiced most commonly replaced by dental and alveolar plosives [ḏ d] and the voiceless most commonly replaced by labiodental fricative [f], which could be an indication of each fricative's acoustic and phonemic representation in each non-native speaker's phonological component, supported by findings of Brannen (2002). Some literature suggests that varying values of [continuant] in speakers' native languages are the means by which speakers choose the replacement sounds they do. However, such an explanation cannot be the only valid one when inherent variability comes into play and different places and manners of articulation are chosen for both interdental fricatives.