X0303 4 2010-07-30 0 0001373671 Isilon Systems, Inc. ISLN 0001030574 FIDELMAN BARRY J C/O ATLAS VENTURE 890 WINTER STREET, SUITE 320 WALTHAM MA 02451 1 0 1 0 Common Stock 2010-07-30 4 S 0 100000 17.4719 D 14240080 I See Footnote Common Stock 2010-08-02 4 S 0 300000 18.1215 D 13940080 I See Footnote Common Stock 2010-08-03 4 S 0 44700 17.6659 D 13895380 I See Footnote Sale of 98,684 shares of the Issuer's Common Stock by Atlas Venture Fund V, L.P. ("Atlas V") and of 1,316 shares of the Issuer's Common Stock held by Atlas Venture Entrepreneurs' Fund V, L.P. ("AVE V"). These shares are owned directly by Atlas V and AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. Represents a weighted-average price of shares disposed. These shares were sold in multiple transactions at prices ranging from $17.40 to $17.59, inclusive. The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the SEC staff, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth. Represents aggregate shares beneficially owned after the reported transaction. Includes 14,052,711shares held directly by Atlas V and 187,369 shares held directly by AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. Sale of 296,053 shares of the Issuer's Common Stock by Atlas V and of 3,947 shares of the Issuer's Common Stock held by AVE V. These shares are owned directly by Atlas V and AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. Represents a weighted-average price of shares disposed. These shares were sold in multiple transactions at prices ranging from $17.74 to $18.68, inclusive. The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the SEC staff, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth. Represents aggregate shares beneficially owned after the reported transaction. Includes 13,756,658 shares held directly by Atlas V and 183,422 shares held directly by AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. Sale of 44,112 shares of the Issuer's Common Stock by Atlas V and of 588 shares of the Issuer's Common Stock held by AVE V. These shares are owned directly by Atlas V and AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. Represents a weighted-average price of shares disposed. These shares were sold in multiple transactions at prices ranging from $17.60 to $17.86, inclusive. The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the SEC staff, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth. Represents aggregate shares beneficially owned after all of the reported transactions. Includes 13,712,546 shares held directly by Atlas V and 182,834 shares held directly by AVE V. Barry Fidelman is a Partner of Atlas Venture. Mr. Fidelman disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. /s/ Kristen Laguerre - Attorney in Fact 2010-08-03