Previous studies have recognized the importance of networks as a channel of knowledge acquisition and have empirically analysed collaborative research networks and the citation networks of articles and patents. However, very little is known about the role of researchers' personal networks in knowledge acquisition. Analysing approximately 2500 ties of approximately 1000 researchers working in Japanese national universities and laboratories, this study revealed which ties in personal networks are used for the acquisition of specific knowledge, focusing on the geographical distance between a knowledge sender and a recipient, the strength of the tie connecting them, and the contents and characteristics of transferred knowledge. This study found that the strength of ties measured by the frequency of knowledge exchange positively affects the acquisition of cutting-edge knowledge and problem-solving knowledge. It is generally known that a greater geographical distance decreases knowledge transfer, and this study found that a greater geographical distance weakens the strength of ties measured by the frequency of knowledge exchange and decreases knowledge acquisition. However, the findings also revealed that cutting-edge knowledge is likely to be acquired from actors who are geographically remote because knowledge is geographically unevenly distributed in the world. By contrast, individuals are more likely to acquire problem-solving knowledge, characterized by more tacitness and context-specificity, from geographically close actors with whom the individual shares context. In addition, the strength of ties measured by not only the frequency of knowledge exchange but also the duration of knowledge exchange in years positively affects the acquisition of problem-solving knowledge. Therefore, the strength of ties is more important for problem-solving knowledge acquisition than for cutting-edge knowledge acquisition. A large geographical distance is likely to decrease the frequency of knowledge exchange, but a greater duration of knowledge exchange can compensate for the lower frequency and can facilitate the acquisition of problem-solving knowledge. This study shows that geographically remote actors and close actors have different roles and suggests that forming various ties and maintaining strong ties, which are likely accompanied by trust/affection, are important for researchers' knowledge acquisition.