Second language research has shown that grammatical structures in the second language are acquired in phases, similar to first language learning. There is a preference for easy, frequently used forms and prevalent archetypes, which are independent of age and first language of the learners. This interlanguage is not a deficit system of the target language, but autonomous and dynamic, developing more and more into the target system. It is characterised by free variation, a term referring to the fact that correct and incorrect forms are being used at the same time. That is to say that high level systems, already close to the target language can exist next to very basic forms. If there is no further progress towards the target language, one speaks of fossilisation. It is not resistant to factors from outside, thus, a break up is possible. This thesis examines the development of the verb system in the learner language of migrants, who attended a three-month German course. The group was very diverse regarding age, first language and duration of stay. Participants were tested after eight, ten and twelve weeks by writing texts about a comic strip. These texts were analysed with regard to the verb system.In the course of the three tests, the number of correct verb forms in the present tense increased, while the number of missing and incorrect verbs decreased. The total amount of modal verbs as well as the number of correctly used modal verbs also increased. The same development could be observed regarding perfect tense forms. In addition, the word order in the present tense as well as in the past tense improved towards the target system. The described process was observed in people who had lived in Austria only for a short period of time as well as in migrants already living in Austria for several years. This result disproves the assumption of a fossilisation, or rather shows the possibility of breaking it up by being specifically engaged with the target language.