Answer:

Megasthenes is the Greek historian who informed us that Bhagavatism first grew up around the Yamuna in Mathura district.

Megasthenes:
- Megasthenes was a Greek historian and ambassador to the court of Chandragupta Maurya in the 4th century BCE.
- He wrote a book called "Indika," which is one of the earliest accounts of India by a foreigner.
- In his book, Megasthenes provides valuable information about the political, social, and cultural aspects of ancient India.

Bhagavatism:
- Bhagavatism refers to the worship of the deity Bhagavan, which is often associated with the god Krishna.
- It is a devotional sect that emerged in ancient India and is an important aspect of Vaishnavism.
- The Bhagavatas believed in the divine nature of Krishna and engaged in devotional practices like singing hymns, dancing, and narrating stories about Krishna's life.

Yamuna River:
- The Yamuna River is one of the major rivers in northern India and is considered sacred in Hinduism.
- It flows through several states including Uttarakhand, Himachal Pradesh, Delhi, and Uttar Pradesh.
- Mathura, located in the Mathura district of Uttar Pradesh, is considered one of the most important pilgrimage sites for Krishna devotees.
- According to Hindu mythology, Mathura is the birthplace of Lord Krishna.

Megasthenes' Account:
- In his book "Indika," Megasthenes mentions the presence of Bhagavatism around the Yamuna in the Mathura district.
- He describes the worship of Krishna and the devotional practices followed by the Bhagavatas.
- Megasthenes' account provides valuable insights into the religious and cultural practices of ancient India, specifically the worship of Krishna in the Mathura region.

Therefore, it was Megasthenes who informed us about the growth of Bhagavatism around the Yamuna in the Mathura district. His account in "Indika" provides valuable information about the devotional sect and its practices in ancient India.