The paper contributes to the debate on the effects that migrants have in the labour market in country of arrival. It analyses the labour supply decision of the migrant worker comparing it with the native one. In a labour market where wage and hours of work are fixed by occupation, a differential behavior in the "effort devolved in the work" is derived by different conditions in the supply. Two cases are examined: the first where different unemployment compensations exist and the second where differences in consumption bundles - with remittances for the foreigner - increase the purchaisng power of the wage. In both cases the migrant optimal effort will be higher than the native one, but through time the foreigner's effort will converge to the native one for the reduction of the initial differential condition. Two types of employers are considered: an high tournover-cost employer and a low tournover-cost one. For the second one will be more efficient to hire migrant as soon as they arrive, before the reduction of the differential in effort, and fire them later, thus explaining the higher migrant unemployment more frequent in the later phase of immigration.