Dr H.B. van Horstok, physician, surgeon and zoologist from Alkmaar in the Netherlands (Pappe, 1866), qualified as Doctor of Medicine (MD) with a thesis entitled De scorbufo (Leiden, 1821, 23p). Soon afterwards he came to the Cape Colony and settled in Cape Town. In 1822 he applied to be registered as a physician in the colony and was duly registered as surgeon and accoucheur (male midwife). He opened a practice at 12 Burg Street, but by 1824 had moved it to 22 Strand Street and by 1827 to 22 Korteman Street.
Van Horstok was one of nine Cape Town medical practitioners who requested the governor in December 1829 to approve the formal establishment of the South African Medical Society (members had been meeting informally for some time). Approval was given later that month. He had been a participant in the earlier informal meetings of the society, for at a meeting on 12 November 1827 he and Dr J.K. van Oosterzee* were appointed to revise or edit some Dutch papers. In 1830 he was involved in a defamation case against the newspaper De Zuid-Afrikaan. He was still medically active at the Cape in 1833. Around that time he was one of the directors of the European Sick and Burial Society. He was a member of the South African Institution, and remained a member for a short time after this society amalgamated with the South African Literary Society in 1832 to form the South African Literary and Scientific Institution.
In 1828 van Horstok requested permission from the government to shoot game in order to provide specimens to the Royal Netherlands Museum of Natural History. Two years later he was still acting as agent for the museum. Among others he sent them fishes from the Cape.