and being under the tutelage of experts. This implies that group-tacit knowledge reflects in the experiences shared on the job and the expertise that arises by reason of learning subtle skills and know-hows that are not common to most members of a profession and, indeed, the organisation.According to Li, Chang, Chen and Jiang (2010) grouptacit knowledge has the advantage of enhancing the effectiveness of organisational members' ability to forecast events relating to their operations. This is because, leveraging on experience and leveraging on collective knowledge sharing they are able to predict future events and thus, ensure effective strategic planning. This view therefore, emphasizes the fact that organisational effectiveness basically resides Abstract. Considering that most part of organisational operations depend on group effectiveness, yet there is scarcely any empirical study on the interactions between group-tacit knowledge and organisational effectiveness. In order to overcome this gap, this study involves a survey of 230 managers and other administrative & technical employees of the four major firm in the Global System for Mobile Communication (GSM) sub-market of Nigeria's telecommunications industry. Data was gathered using a mixed method research approach. Correlation, Regression, Analysis of Variance (ANOVA) statistics and thematic analysis was carried out as means of testing the hypothesis for this research study. The findings showed that firms need to focus keenly on ideas of employees that are shared during conferences, team leaders' mentoring role, collective reasoning over task and constantly hold strategic meetings that probe into developments arising in each functional unit of the organisation. These factors are shown to have a strong influence on organisational effectiveness.