Net sales of unit trusts and Peps fell to £149.4m in September, down from £421.4m in August, according to figures from the Association of Unit Trusts and Investment Funds.
The main cause was lack of buying by the institutions, which had net sales of minus £184.2m.
Gross institutional sales of £957m were slightly down on last month, but a record level of repurchases from the UK Smaller Companies sector pushed the overall net position to a negative.
Repurchases in the Japan sectors also far out-weighed sales, to contribute to the low net total.
There was a slight pick up in net Pep sales in September, despite the FTSE 100 and other international markets showing continued volatility.
Net Pep sales were £406m, up from £400m in August and £404m in September last year.
Autif director general Philip Warland says the figures are pleasing considering the market performance.
He says: "This latest set of figures is our strongest evidence yet that private investors buying investment funds, especially via Peps, are committed savers, and understand that investment funds offer the key to long term financial security."