The  present  study  proposes  the  analysis  upon  the  rationalities  that  guide  the production conducts of modern family farmers who foster different adjustments in the production systems through  their social  sustainability  practices.  The  study  of these conducts was carried out based on their social practices,  always from the axis of the relations  established  between  the  farmers and  the  market  as  far  as commercialization, technologies and sociability are concerned. Two hypothesis were assumed:  the  first  one  is  founded  on  the  idea  that  modern  family  farmers make different rationalities in order to guide their conducts, and these rationalities are the result of  significations  which  are  based  upon  social  sustainability;  the  second hypothesis suggests that the construction  of farmers' social identities is defined  by these rationalities. The approach perspective was basedon family farmers. So as to better understand these social conducts, this study was grounded on Weber's ideal types,  and  that  made  it  possible  to  substitute  the diversity  of  conducts  for  an intelligible,  coherent  and  rational  material.  The  social  universe of this  research involved two political regions of Paraná: southwest andwest, which are a continuum space where family farmers' establishments and productive  heterogeneity dominate. In  order  to provide  information  gathering,  interviews  were  conducted  with  farmers and qualified informants with the use of semi structuredscripts. Weber's concept of social  rationality  was  used to  comprehend  farmers'  social  conducts.  Men  attribute meaning to their conducts when they create values, and the values that guide their conducts  are  expressed  in  rationalities.  The results  of  the present  study  made  it possible  to  determine  that  farmers  build  different rationalities  and  that  the (re)construction of their social professional identities  are defined by the rationalities adopted by them, which confirms the initial hypothesis herein stated. The rationalities that guide farmers conducts strongly contribute to the construction of the concept of family  farming;  farmers  can  also  be  defined  by  their  rationality  regarding  the interaction they  establish  with  the  market.  Hence,  this  specificity  concerning  family farmers calls for State public policies that are characteristically more specific and less universal.