This paper investigates the nature of the substitutions used for the dental fricatives (/theta/ and /eth/) by Dutch learners of English as a second language. By means of an OT analysis, the underlying reasons for the difficulties encountered with these sounds are brought to light. The present data reveal that phonetics (or acoustics) rather than phonology plays a major role in the selection of the phonemes used to substitute the dental fricatives. (Contains 5 tables and 1 figure.)