Autism spectrum disorder (ASD) is a neurodevelopmental disorder, characterized by social communication deficits and atypical patterns of behaviour, interests and activities. Although it was previously considered that the language deviation was an important clinical sign and symptom of the ASD, nowadays researchers agree that the population of children diagnosed with ASD can be divided into two subgroups, depending on the results of language tests and measures: the ones with accompanying language deviation and those without it. Considering the fact that the latest researches clearly show that the language typology can influence the language acquisition, as well as the range, quality and number of errors, the first goal of this work was to examine the ways in which different morphological complexity of English and Croatian influences the quality and quantity of morphological and morphosyntactic errors made by children with ASD from Croatian and English speaking areas. The second goal was to examine the homogeneity of language phenotype in the subgroup of children with ASD who show associated language deviation. Individual therapy sessions of two Croatian speaking five-year-old girls diagnosed with ASD and language deviation were recorded during a five-month period. The recorded speech sample was transcribed in application CLAN, which is a part of the CHILDES system (MacWhinney, 2000). The errors produced by the girls were classified and quantified using the Microsoft Excel (2010) software. The results have shown that the examinees produce multiform inflectional and derivational errors, as well as the overgeneralization toward simpler language constructions. By comparing these results with Anglo-Saxon literature about errors among English speaking children with ASD, it becomes clear that the errors occured at many different levels in Croatian compared to English, and that the errors appeared in all word classes and grammar categories. Based on these facts, it has been proven that the greater morphological complexity of Croatian language causes more numerous and diverse types of errors among children with ASD from Croatian speaking area. Regardless of their language development level, both girls produced errors of the same type. However, a few differences were found in the quantity of specific errors in certain error types. These findings are important because of the insufficient researches about language characteristics of Croatian speaking children with ASD, and can also serve as a guidance in creating language support system for children with ASD.