Gathering data from another sheet.

Hello,

I have a sheet that refers to another sheet in the workbook, lets call them sheet1 and sheet2. On Sheet2 I have referenced a cell in Sheet1 by using

Formula:

=Sheet1!A1

If I wanted to copy the formula down by increasing the row by 20,

Formula:

=Sheet1!A21

is there an easy way to "Fill Down"? The other option I have is to manually refer the cells around 3,000 times. Thank you in Advance.